You can't interupt an attack string and enter an stance or anything simulair if the other person don't know the string off attack or if he/she is just super agresive. I've had the same problem as Taki by doing BA PO P wanting them to think that i'm gonna do BAA and duck for the last hit when the last hit in BA PO B is an mid attack. Buttom line is its easier to trick someone decent then it is to trick a masher/spammer/scrub or whaterver you want to say.