The Mega Arm

Mega Man's charge shot is replaced by firing his entire fist.

Charge Shots

A "half charge" is about 30-89 frames of holding B. A full charge is then 90+ frames.

Half charges deal 2 damage, full charges 3. The fist after that returns to Mega Man, and the behavior is the same.

Snapback Damage

When Mega Man's fist returns, it can damage enemies it touches on the way back. This can be used to do extra damage to enemies, notably the end of Mercury's stage, as the fist deals as much damage as the initial charge shot (typically 3).