Overpot Zerking

I cant say for all skills, but sin's drain for example gets HP back as you cast it. In fact, many skills work that way. Sometimes, I throw TT from a far distance and I see the exp comes up before it fully connects.

That's primarily something that is due to the connection between client and server. From the moment you hit your attack key, the server registers how much damage you did and instantaneously issues that damage to the monster. The animations for something like TT can't possibly keep up with that, so that's why you'll see the exp beforehand. Same thing with Drain.