A shield flinch would work like this:
If a monster's attack is shielded at the right time(right before they attack), the monster would flinch(and take damage, and the shield would take no/less damage???). If a bullet is shielded at the right time, the bullet would be reflected off the shield and it would deal less damage than/the same damage as it normally did.
This would be a fun mechanic.
Thing with this game is the fact that timing is hard with all the lag that just about everyone suffers from. A timed shield (unless given a considerable amount of time) mechanic is sort of a no-go.
Now if we did get it down well for everyone, reflecting a bullet I'm gonna say no to... However a parry could be neat. Knocking down the monster or absorbing half damage plus shield bumping them would be fun for sure.