That's the counter on shake: back, forwad, 1
If you use it as a projectile is coming towards you and you use it, you will slam down on your opponent. This slam is COMPLETELY inescapable, it will always hit the opponent.
If you perform the enhanced version, it will activate against: Projectiles, highs, overheads, throws, basically everything that's not a low or an x-ray.
You can hold 1 to maintain the counter for up to 5 seconds on the normal or enhanced version. Abuse this move.