This is a problem I like to have. A horse that goes when you touch them. but it can definantly be a little frustrating when your trying to use leg cues. Um what I have found is that the more you stay off a horse with cues the more sensitive they become (and I use this quite a bit on a horse with not very much go when trail riding). However when you want the horse to desensitize then you want to use your legs a lot. For a while this might seem like the horse is going nuts and you will never get them used to it, but then all of the sudden they come around. For example from walk to trot I would bump the horse firmly to go, when then takes off too quick, stop and back them (or whatever you use to reprimand) then ask again until they get it right. I know the previous post said to keep pressure on until they calm, but that is a lot harder said than done when they are trying to take off with you, however it would probably get the same end result. One thing that also helps a lot is a precursor cue, like turning your toes out and then cueing. Why that works i think, is that usually the horses that are very sensitive are also very willing so a precursor helps them prepare for what your about to ask instead of you bumping them and them just reacting to quickly. They want to try so hard for you that they are overachieving. So a precursor makes them slow down and think. Also if I side pass instead of adding pressure with the right leg to move left I would first lift my left leg off the horse a little or just turn my toes out first like stated before.