I have not done h/j since I was a kid, but, I saw a fantastic episode of Clinton Anderson working with a New Zealand TB who was hot and rushing jumps. What he did was so remarkably simple and made so much sense. He would periodically just shut the horse down about a stride before the jump (by shutting down, I mean a one rein stop and hindquarter disengagement). Then he would go back to doing some flat work for a few minutes, let the horse take a jump or two, and then shut him down again before another jump.
The horse very quickly realized that when he was headed towards a jump, he may or may not be shut down. He began to approach the jumps very carefully, obviously paying close attention to Clinton to see if he was going to be allowed to go over a jump. That is exactly what you want in a horse that is rushing jumps.
This was a horse in with top notch trainers, and they had been fighting him for ages, using running martingales, stronger bits, see sawing on him as he was approaching to try to get him under contol. That simply never solves any problem, it just winds the horse up more. With Clinton's idea, you just need a regular snaffle bit and a horse that will give laterally to it (if the horse won't give laterally there is no way he should be jumped anyway, he is not completely broke).
Obviously this is not something you would practice on a horse that is refusing jumps. It is something that is really only used for a horse that is excitable or rushing at jumps.