I've only heard this in the context of Smash Melee, which has limited-to-no input buffering. If you try to input your attacks and movements too soon, they won'

I've only heard this in the context of Smash Melee, which has limited-to-no input buffering. If you try to input your attacks and movements too soon, they won't come out at all. So advice to new players is to err on the side of being slow, since it ensures the move input gets registered.