I'm not actually nearly strong enough to hit high enough numbers to confirm the following, but what I've been told is that countdown zero has two effects - 1. doubles the burst cap (so if you max out exceed, it's 2x 2 mil = 4 mil cap, at zero exceed it should be 2x 1 mil = 2 mil cap) 2. boosts the burst dmg multiplier, and that boost is outside of the normal 500% cap (I'm not sure what that multiplier actually is, but let's just say it's noticeable, lol.)
EDIT:
Actually, JP wiki suggests that the countdown zero multiplier is 450%. Given that AW himes' base burst multiplier is 5.5x, if you max out the 500% burst multiplier, Meta AW will hit 550%+500%+450% = 15x burst multiplier at countdown zero.