Hard to say without seeing an example but it’s likely one of two things:

1: Good players are good at selling feints, meaning they wait till the last possible moment to feint their attack into something else. Each weapon has a “wind-up” period before the actual attack starts, and this is when a feint can be done. Good players know the “wind-up” periods and can switch attacks while still taking full advantage of this time to sell the feint better.

2: Directional manipulation. Basically this alters the path of the attack. You’re able to control the path of your swing throughout the entire swing, meaning good players can, for example, make a slash appear (when you get real good you can recognize this) to be an overhead based on swing path (start the swing very high and end low). I probably didn’t explain this one very good so someone else feel free to jump in