If you have a kimberwick uxeter, you can use the top hole and it really doesn't ever activate the curb chain. But if you use double reins it does. Some horses are very hard mouthed and can't be worked back down to a snaffle. Always ask your trainer/helper their opinion on it. If this is the case with your horse, there are a bunch of different options. You can use a lower port, a broken mouthpiece in the middle, a solid mouthpiece, a medium port. It all depends on what your horse is used to. I've heard some people say that you should switch to a stronger mouthpiece for a few days to get the horse to respect the bit. I've never done that before and have no experience with it whatsoever. But please, always ask someone whose more experienced than you. Some horses just don't go good in kimberwicks because of the chain action. I don't like kimberwicks, I like kimberwick uxeters.