That little Yoshi head flip is actually when you flick the shield button, so that you don't go into your egg-shield. You get a frame of invulnerability (except to throws) during that head flip. That's the parry. I guess it looks like an up-tilt.
But as far as I know, Yoshi's up-tilt doesn't have invulnerability frames.
There's also an effect (probably directly related) where Yoshi gets no shield-stun if he times his egg-shield just right. It's much like power shielding in ssbm, but it doesn't reflect projectiles. Also, it's easier to do this technique accidentally, compared to the parry (at least in my experience
).
I think ant-d has frame data on these things somewhere.