It’s not just something Chun can do. On wakeup, every character except Chun has 1 frame in their animation where even if they’re holding down, they appear standing and can be hit by moves that would not normally hit crouching. The trick with Chun involves doing s.jab -> c.jab so that the first jab hits on this frame. If they’re holding down back, they still get to block, so it’s nothing amazingly broken like getting to do free low attacks on people as they’re getting up, but it does start her mix-up game, relatively safe if the first jab is parried, etc. But the 1 standing frame thing is universal (except for Chun), which is why you can do things like Ken’s far s.fierce -> Shippu link on Ryu if he wakes up in a crouching state, but normal crouching the fierce will whiff.