See explanation on the sidebar.
Edit for important info:
Well, there’s actually two ways to do it.
Press MP+MK, lift your fingers, and stance change. As I say in the explanation, it’s relatively lenient with the stance change timing. If you time your focus a little early this way though, you’ll get a whiffed level 1 focus.
On consecutive frames, do a focus and then finish a stance change. I thought of LP+MP+MK~~HP too, and it seems like the best way. Button shortcuts would work too. This method would give you a stance change animation if you don’t absorb anything on the 1st frame of the focus.
Either way, you’re going to take a hit if you time your focus too early. I think both methods could have advantages though in different situations.
Doing it against Viper or Boxer actually seems like it would be hard to me. The problem is that you need to know EXACTLY when an attack will hit you, so it wouldn’t work against attacks with unpredictable timing.