Ftilt (1st hit) trajectories: Standard flinching?

[BND]

Sometimes, when I Ftilt a grounded opponent, they simply flinch in the same way that Brawl Snake's ftilt causes characters to flinch. I've done a lot of testing with ftilt, but it seems that if ftilt is crouch cancelled it generally knocks down, and most other DIs just result in the standard popup. Does anyone know if there's a specific DI the opponent has to use in order to cause them to not get knocked down/popped up, or is this because of other factors? It has fixed knockback, so I can't see what else can be causing it.