Eesh a shock collar? That's weird for training...
There was one instance where I actually put my dogs shock collar on one of our horses. He was abusing our filly really badly. She was upset seperated from him and our other gelding, but got beat up with him. It was this whole big drama and they never settled their dispute. She had gashes, hoof marks from getting kicked, etc. When I was there he knew better, but I couldn't hang out in the pasture all day. So I put the collar on him (it was designed for tiny dogs, so it had low voltage...and I had to attach extra normal collars to it to lengthen it.
In any case, it was a lower voltage than an electric fence, and we do it to dogs and don't think its mean, so I thought I'd try it. I could see them from the window and when he'd chase her and corner her and start trying to abuse her I'd push the button. He thought SHE did it to him! I honestly think he thought that she'd kicked him, and after a few days he stopped. I took the collar off and never had to put it back on. (I think he had aggression issues. If one of the other horses rolled, he'd attack them while they were on the ground, very angrily. He had a screw loose somewhere. He also tried to kill cats...never dogs, only cats.)
Anyways, I personally think trick training is a great way to teach your horse something fun and keep his mind fresh as long as you take your time and make sure you follow the instruction of a professional who knows how to trick train (because lets face it, if your horse starts laying down like you taught him in the middle of a horse show you might have some problems. Lol)