The input for this move has to be executed extremely fast. If you messed it up, it's because you were too slow. I'm talking consecutive frames fast.
The best way to practice this is to crouch and just put in the aB as fast as you can. This way you eliminate any motion on the joystick and learn just how fast the button press has to be.
The only way I can perform this move consistently is to place my thumb on one side of the A button, then slide it as fast as I can across the controller, hitting A and B in succession in the process. It's fast, and even that's not reliable. Remember that you must release A before hitting B for this to work. If the game registers that B is pressed while A is still held, the input will fail.
Strategically, unless you plan on going to high-end tournaments, I don't think this should be a move you add to your arsenal. The input timing is just too strict and the move is physically difficult to execute. You're probably better off looking for another quick Yoshimitsu move. On the other hand, tournament players consider this one of Yoshimitsu's bread and butter moves, so if you really want to get to the top of your game, prepare to spend quite a while mastering this.