So shield advantage is the amount of "advantage in frames" if you land the attack and auto cancel it?
Shield advantage doesn't necessarily have anything to do with auto-cancelling an attack; it
might, but it doesn't have to.
This is the basic scenario:
Your opponent has their shield up.
You hit them with an aerial.
Your opponent goes into shield stun.
You go into hitlag (?), which is always (?) shorter than their shield stun.
You then land, and either suffer:
. 1) normal landing lag if
miss your L-cancel
. 2) halved landing lag (rounded) if you
do L-cancel
. 3) auto-cancel landing lag if you hit the ground during the auto-cancel frames of your aerial (for Bowser's nair, this is if you land on frames 32 to 52 of the move)
Your shield advantage is then calculated as how many frames you regain control of your character (meaning you can begin to enter an input and have it actually do something) earlier than your opponent regains control of their character. Positive means you can begin to act before your opponent, negative means you begin to act after your opponent.
It depends on a lot of factors, such as:
1) what type of landing lag you suffered
2) how long their shield was in shieldstun (which can be affected by move staleness)
3) how long you were in hitlag (I'm not actually sure what this depends on)
4) and how much earlier your hitbox hit their shield before you entered your landing lag (if Bowser's nair hit the top of the opponent's shield and then he had to take a while to fall to the ground, his shield advantage would be more negative than if he hit the opponent's shield the frame before he hit the ground)
For example, if you spaced properly and hit your opponent's shield with nair's hitbox during one of the frames between frames 15 to 28, then hit the ground immediately on the next frame with L-cancelled landing lag, you would be -6 on their shield, meaning your opponent regains control 6 frames earlier than you do. In most cases, you either hope you spaced it well enough that your opponent can't punish you (doesn't seem possible with nair, another move might be better, like fair), depend on the human factor that they can't
immediately act out of shieldstun (apart from buffered rolls, it's very hard to do), or throw out an up-b and slide away (it's much easier to tell when
you regain control of your character than when
they regain control of theirs, since you're so used to your own moves).
Edit: I got ninja'd.