This video explains how to do an Active Switch (AS) cross-up in BBTag, how to fake the cross-up, and important info to know to make sure you do these mixups correctly.
The inputs for the AS cross-up are:
5P 5A j9 IAD (land behind them) 214B+D (release D as fast as possible to AS)
An alternative way of inputting the AS cross-up, instead of pressing both B and D at the same time, is to plink from D to B: 214D~B. This makes it easier to release D sooner after pressing B. If you release D too late, after 214B hits the opponent, then it will not cross-up.
(source: Bill307fighting on YouTube)