Signia
[13] Hero
Actually, the axis doesn't matter. 663A+B works just fine while QS~66A+B does not. They're different moves, the difference being either the stun animation, or the recovery is different.They both are the same move, but 33A+B hits the opponent to where Yoshi is slighty on a different axis so that stun allows to iMCF combo easily.
The damage you get from 33A+B into iMCF is huge, but it's inconsistent as a sidestep whiff punish because it's slow and sometimes whiffs, especially up close, like 22B.
The best options for whiff punishes depend on range: 3B is really consistent when it reaches, 22B is consistent outside that range, and 33A+B is good for ... well I haven't had any success with it yet, I'll have to try it out.
backstep~4123 A+B would probably be good for backstepping and punishing throws... I already do 4~412B for backstep 22B.