Actually best of envelop is you can do it on reaction.
Take for example soph/mitsu 1A,A (2B+K) or 1A,B (B+K).
Then there are lot of strings on wich you can answer both mixups with B+K talim AA.. combos (from 4th or third move) or again Nightmare flashy terror charge combo TT even if it seems to stun on blok you can actually press B+K on second hit.
I suggest you to practice against human opponents… its easier than it seems.
P.S: not really sure but i'd bet it starts frame 4