How come I've never tackled this already? Ahh well, let's get to it.
In short, the concept of Early Guarding is to literally guard early while the opponent's power lines are still developing. At the cost of setting up your Counter-Blasts earlier on in the fight, you take advantage of the opponent's generally weak field to stave off attacks with cheaper amounts of shielding, allowing you to tank through the game with even less effort than to try guarding out huge attacks later on.
Of course though, this does bring up the question, what defines guarding 'cheaply'? In fact, let's take this a step further, and figure what's the absolute cheapest you can guard for. In terms of raw advantage, the answer is quite obvious. The very least you can guard an attack is for 1 card. In terms of auxiliary advantage now, ignoring Grade 3s and Perfect Guards, the 2 printed numbers are for 5K shield and 10K shield within just a single card. To sum up, the absolute cheapest you can guard for is a single 5K or 10K shield.
Interesting to note, if you can guard all 3 of the opponent's attacks for a single 5K shield each, Grade 3's draw 3 cards per turn, and considering that the average shielding in most decks is about 5K per card, you can theoretically sit on a pure stalemate until either player decks out. Of course though, this situation is so improbable, I'd go as far as to say it's impossible. Ahh well. This in mind though, it really should highlight for you that guarding for the absolute cheapest way possible is beneficial at extending the amount of time you can build up your forces throughout a fight.
In a sense, Guarding Early is the perfect sense of breaking even with the opponent's advantage through battle, and not allowing them to get an edge over you. To sum up, one should almost always take opportunities to guard for cheap amounts to generally save their shielding over the course of a match, and allow you to break even with the opponent's advantage through battle. Of course, there are ways to turn this entire system on its head, and here's where I talk about pressure units.
All the way since Dragonic Overlord pressure units seemed to have generated some of the biggest misconceptions in the entire game simply due to the nature of how on-hit* skills work. The general consensus is that when an on-hit skill unit attacks, after usually paying costs, they generate some form of Card Advantage for the player to utilize. From another attack to unflipping damage to drawing more units, Pressure Units offer an amazing amount of utility to a deckbuilder at hand. But what gives? Most on-hit units can't attack for more than 20K on their own, and didn't I just take a lengthy part of this article just to explain that guarding for a single 5K shield is a break even, and a single 10K shield is the next best thing? How can pressure units who can so easily be blocked out like that be any use to you?
*Skills that gain an extra Critical also generally fall into this category
Well remember what I said in detail. Forcing out a single card, whether a 5K or 10K shield, is virtually a break even in card advantage, right? Well what if you can force more than that. If a single 5K or 10K shield is a break even, then we must view anything more to be forced out as a net gain for the attacker. This means that if a pressure unit can attack to force out at least 15K shield, then they have still done their job at gaining you advantage from that attack.
But there's still one problem, the majority of pressure units still can't attack for higher than 20K on their own. Of course though, we can always turn to triggers. These next numbers will assume that both players are running a 12 Crit 4 Heal Set up:
Player 1 pulls 0 Triggers: (22/49)
Player 2 pulls 0 damage triggers: (33/49) - 726/2401
Player 2 pulls 1 damage trigger: (16/49) - 352/2401
Player 1 pulls 1 Heal Trigger: (11/98)
Player 2 pulls 0 damage triggers: (33/49) - 363/4802
Player 2 pulls 1 damage trigger: (16/49) - 88/2401
Player 1 pulls 1 Critical Trigger: (33/98)
Player 2 pulls 0 damage triggers: (22/49) - 363/2401
Player 2 pulls 1 damage trigger: (22/49) - 363/2401
Player 2 pulls 2 damage triggers: (5/49) - 165/4802
Player 1 pulls 2 Heal Triggers: (1/196)
Player 2 pulls 0 damage triggers: (33/49) - (33/9604)
Player 2 pulls 1 damage trigger: (16/49) - (4/2401)
Player 1 pulls 1 Heal Trigger and 1 Critical Trigger: (2/49)
Player 2 pulls 0 damage triggers: (22/49) - (44/2401)
Player 2 pulls 1 damage trigger: (22/49) - (44/2401)
Player 2 pulls 2 damage triggers: (5/49) - (10/2401)
Player 1 pulls 2 Critical Triggers: (11/196)
Player 2 pulls 0 damage triggers: (682/2303) - 3751/225694
Player 2 pulls 1 damage trigger: (1056/2303) - 11616/451388
Player 2 pulls 2 damage triggers: (495/2303) - 5445/451388
Player 2 pulls 3 damage triggers: (10/329) - 110/64484
Now that just doesn't look nice. With barely over a 3/10 chance to gain a + every turn, maybe people were right. Wether you think the glass is half full or empty (or I guess 3/10 in this case), the fact remains that as a source of card advantage, and because of how fragile a Rear-Guard can become if the opponent focuses on gunning it down, this sort of card advantage just isn't practical. Because the opponent can pull damage triggers as you Twin Drive, the advantage from these temporary cards just isn't practical. But what if you can eliminate this entirely? You see, all these numbers are based off probabilities of your Rear-Guards attacking to force out 15K shield in order to generate advantage, but a Vanguard with a valuable pressure skill like Amber Dragon, Eclipse or Stern Blaukruger will have access to their triggers before the opponent can damage check if they attack first. In fact, if they are properly boosted and the opponent has to guard, that's 15K shield right there. That's an immediate + with that single attack if they are forced to guard and continuously so until they either can't pay for their on-hit's costs and/or once you hit the 4 damage mark (kinda). Even worse is if you can double up a Vanguard pressure attack with a Rear-Guard pressure attack. Without damage triggers getting in the way from then on out, you are looking at +1 from the opponent if they are forced to guard the Vanguard, and (on average) an extra +.55 from needing to guard a trigger boosted Rear-Guard. Basically a +1.5 turn per turn if you can set both up, and this is ignoring the off chance they hit and their skills generate advantage.
Now it's not all fun and games for on-hit units, namely as soon as you reach the late game. While yes, in the late game the same rules apply in terms of advantage through battle, namely that guarding for 15K or more is still a minus in some form, the fact remains that if the opponent is in high damage, they will be extremely inclined to guard out your attacks anyway. Unlike in the midgame, when the opponent would have had no need to guard 15K attacks otherwise, on-hit skills don't force the opponent to do something that minuses them any more than a non On-Hit unit can do in the late game, meaning that the advantage an on-hit skilled unit generates is almost entirely shut down during the late game, assuming your opponent isn't a scrub who eats damage just to eat damage, that is. To make up for this, decks that utilize pressure units generally need to partner up with a more explosive unit or entire deck line up to make the most card advantage in a short time span within the late game. This in mind though, I need to highlight a recent development of units: On-Hit Limit Breaks. Honestly, I have no idea what's going on with those sort of cards, and almost failing as a duality of mechanics. A skill that's best used in the Early/Midgame in tandem with a requirement that forces it to go off only in the late game where its effectiveness almost totally dies off? It's just not practical, or even useful in a great majority of cases to you.
Now I'm going to pretend I didn't mention Early Guarding just to use it as a springboard to explain the technicalities behind on-hit skills, and start diving into ways to help fight against on-hit skills and to keep the advantage they can generate in check.
Probably the first thing that comes to mind, if you're facing off Rear-Guard with on-hit skills, it should become pretty high priority to get rid of it really quickly. Generally, the longer an on-hit skill unit is on the field, the more and more opportunities it gets to wreck your hand, so you should direct your attention to those that are called earlier on in a fight. Sadly, unless you are running a retire based deck to deal with the opponent's Vanguard booster, it is near impossible for a great majority of decks to incapacitate an on-hit Vanguard's skill, making this a near irrelevant argument against them.
Oh hey, passive aggressiveness. Here's where you play the numbers game. When dealing with an on-hit skilled unit, you need to keep in mind what kind of effect on the game its skill will have on you. Okay, it has a draw skill, but is the opponent in need for fieldable units, or does the deck have the Rear-Guards that the opponent should be digging for? Is it going to cost me more to just take the hit now or have to guard out attacks from it from here on out? Situations like these inevitably come and go, and require a lot of quick thinking on demand to really be utilized well, however it's the simplest way to deal with pressure units if any other option is out of your reach at the moment. The slight flaw in this though, is that on-hit Rear-Guards can't generate much advantage on their own so guarding them out would probably already have been on top of your list of options, and many Vanguards have extremely detrimental skills to the opponent should they hit, meaning you're near forced to guard them anyway, essentially making this course of action moot in many cases.
More associated to the cards that directly affect Counter-Blasts, the final general way to help you jam Pressure units is to watch how the deck works. Those who have seen the Stern Blaukruger decklist will probably get this right away, as Magician Girl Kirara's Counter-Blast cost is bound to get in Stern Blaukruger's way at some point. This in mind, an opponent who hits you with their Kirara may not actually set off its skill, in fear of jamming Stern Blaukruger. Because of this, it generally allows leeway for your hand to simply let those attacks through, knowing the opponent may not benefit from it much anyway. This kind of thought doesn't especially work if the on-hit skill attacks to deal the 4th damage however, and is highly reliant of deck choices clashing in the first place.
To sum up, Early Guarding is one of the best ways you can create a buffer for yourself from reaching a dangerous level at 4 and more damage by using total break evens in single 5 and 10K shields to keep your card advantage in tact, while your damage remains well paced. However, abusing that system through methods like on-hit skills allows an opposing player to start gaining net +'s by using those kinds of units to force you to drop more shield than normally necessary. While checks and balances exist for those kinds of units, the amount of card advantage they can generally create when in full stride is substantial, and can be extremely threatening if let loose.