Shieldstun, shieldhitlag, and frame advantage
When an attack hits, there's an amount of time where the person who shielded can't do anything (shieldstun) and the person who attacked will be in freeze-frames (shieldhitlag). To determine advantage, you take when the attack hits, add the shieldstun, and then subtract the shieldhitlag and total framecount. In other words,
HIT + SHIELDSTUN - SHIELDHITLAG - TOTAL = ADVANTAGE
Advantage is typically negative in Brawl, from what I can tell, so don't be too disappointed.
Human Reaction
While people can properly time things and react as quickly as 0.1 second, or 6 frames, after something hits, a normal human will tend to react in 0.2 second, or 12 frames, to something they don't expect. Add these 12 frames to your advantage value to get a "virtual" advantage value. If you end up with a positive number, the attack is pretty much safe.
If you feel like further adding numbers to make you feel better, it takes 4 frames to put away your shield, so add +4 if your opponent wants to do something involving unshielding and attacking.
Unsafe Attacks
So we'll go with the obvious example of something unsafe on block, Marth's fsmash.
TOTAL: 50
HIT: 11
SHIELDSTUN: 13
SHIELDHITLAG: 4
ADVANTAGE: -30
VADVANTAGE: -18
In other words, after shielding the hit the opponent will have 30 frames (half a second) to punish you before you can do anything about it. Even factoring in human reaction gives them an 18 frame (.3 second) window to unshield and do something.
Deceptively Safe Attacks
Now, let's take a look at landing a tipper hit with an fsmash.
TOTAL: 50
HIT: 12
SHIELDSTUN: 14
SHIELDHITLAG: 6
ADVANTAGE: -30
VADVANTAGE: -18
Wait, what? It's the exact same as a non-tipper! However, this is at tipper range and the shieldhit will cause the opponent to slide back, forcing him to waste some time running back up to you. Doesn't really help against projectile users, though.
Safe Attacks
And of course, Marth's amazing dtilt has to come under scrutiny.
TOTAL: 22
HIT: 7
SHIELDSTUN: 12
SHIELDHITLAG: 5
ADVANTAGE: -8
VADVANTAGE: 4
An actual safe move, unless your opponent sees it coming and reacts quickly enough. If he doesn't, you're gaining +4 frames on him with this attack on his shield.
Powershield
Watch out for the powershield, though. The behavior is as follows:
No shield damage
Shieldstun is reduced to equal that of shieldhitlag
Character does not get pushed back by the attack at all
Shield is not actually activated, meaning they don't get the 4-frame loss of unshielding
You can look at it as basically your attack completely missing them. It's impossible to powershield multihit attacks in Brawl, unfortunately (unless you click that shield trigger for each and every hit somehow), but that's fine because powershielding anything with Marth will give you an adequate window to counter with Up+B any time.
Here is what happens to an fsmash tipper that gets powershielded:
TOTAL: 50
HIT: 12
SHIELDSTUN: 6
SHIELDHITLAG: 6
ADVANTAGE: -38
VADVANTAGE: -26
...Yeah, don't spam smash attacks.
Hope this helps.
When an attack hits, there's an amount of time where the person who shielded can't do anything (shieldstun) and the person who attacked will be in freeze-frames (shieldhitlag). To determine advantage, you take when the attack hits, add the shieldstun, and then subtract the shieldhitlag and total framecount. In other words,
HIT + SHIELDSTUN - SHIELDHITLAG - TOTAL = ADVANTAGE
Advantage is typically negative in Brawl, from what I can tell, so don't be too disappointed.
Human Reaction
While people can properly time things and react as quickly as 0.1 second, or 6 frames, after something hits, a normal human will tend to react in 0.2 second, or 12 frames, to something they don't expect. Add these 12 frames to your advantage value to get a "virtual" advantage value. If you end up with a positive number, the attack is pretty much safe.
If you feel like further adding numbers to make you feel better, it takes 4 frames to put away your shield, so add +4 if your opponent wants to do something involving unshielding and attacking.
Unsafe Attacks
So we'll go with the obvious example of something unsafe on block, Marth's fsmash.
TOTAL: 50
HIT: 11
SHIELDSTUN: 13
SHIELDHITLAG: 4
ADVANTAGE: -30
VADVANTAGE: -18
In other words, after shielding the hit the opponent will have 30 frames (half a second) to punish you before you can do anything about it. Even factoring in human reaction gives them an 18 frame (.3 second) window to unshield and do something.
Deceptively Safe Attacks
Now, let's take a look at landing a tipper hit with an fsmash.
TOTAL: 50
HIT: 12
SHIELDSTUN: 14
SHIELDHITLAG: 6
ADVANTAGE: -30
VADVANTAGE: -18
Wait, what? It's the exact same as a non-tipper! However, this is at tipper range and the shieldhit will cause the opponent to slide back, forcing him to waste some time running back up to you. Doesn't really help against projectile users, though.
Safe Attacks
And of course, Marth's amazing dtilt has to come under scrutiny.
TOTAL: 22
HIT: 7
SHIELDSTUN: 12
SHIELDHITLAG: 5
ADVANTAGE: -8
VADVANTAGE: 4
An actual safe move, unless your opponent sees it coming and reacts quickly enough. If he doesn't, you're gaining +4 frames on him with this attack on his shield.
Powershield
Watch out for the powershield, though. The behavior is as follows:
No shield damage
Shieldstun is reduced to equal that of shieldhitlag
Character does not get pushed back by the attack at all
Shield is not actually activated, meaning they don't get the 4-frame loss of unshielding
You can look at it as basically your attack completely missing them. It's impossible to powershield multihit attacks in Brawl, unfortunately (unless you click that shield trigger for each and every hit somehow), but that's fine because powershielding anything with Marth will give you an adequate window to counter with Up+B any time.
Here is what happens to an fsmash tipper that gets powershielded:
TOTAL: 50
HIT: 12
SHIELDSTUN: 6
SHIELDHITLAG: 6
ADVANTAGE: -38
VADVANTAGE: -26
...Yeah, don't spam smash attacks.
Hope this helps.
