i found this on the assassins forums if its a bug or not im not really sure but it sounds useful
Pressing active block right after any combo step except the last will cancel the last key press in the combo. For example, the combo is UL, UR, M. I press UL, then UR, then X for active block. UR will then fade out and I get to press it again. This also works if you press the wrong key. For example, I press UL, then M by accident. If I immediately press X after M, the combo will back up a step, allowing me to correct my mistake.
A second use of this is to "hold" a combo. This is because it is possible to keep a combo hot indefinitely by canceling the 2nd to last key press over and over. You can use this to buy yourself time as you find an opening for a large combo like SSX or SDS.