My cousin tends to do a whiffed demon flip palm as an okizeme after he lands what IIRC is a hp shoryu, lands behind me, DEMON!
When I try to do it, I try to use it as an anti-air by lowering my hittable box with sweep, while they're coming down and me low to the ground, I input demon, their limb passes through my invincibility frames, they die 1000 deaths.
To expand on hit-confirms, I've sure you've seen pros play and they do all these combos... while you're performing the combo, there's more than enough time for you to know that they're in a state where you can keep hitting them. With that in mind, the idea is to land strong attacks afterwards (i.e. special moves). for example, let's say you were trying to land a dragon punch. You're not going to walk up to them and do it; they can block that. What people do like to do is string together some normal moves to see whether or not they were blocking or not. If they weren't, the attacking player follows up that first normal (let's say it was a crouching jab) with another jab. At this point you know that they've been opened up (defence has been broken and now you're deal as much damage as you want), link that last jab with another normal perhaps (like a cr.mk or cr.mp), cancel that into a special move (i.e. dragon punch, tatsu, hadoken) because you know now that your special move won't be blocked. 👍