I'm not sure if you have ever heard of the see saw method? But I just put pressure, release, pressure, release. Posting also helps
Oh wow. No. Please don't seesaw your horse's mouth. It is both potentially dangerous as well as detrimental to your horse's training. Seesawing on a horse with a sensitive mouth could cause the horse to react in a way that could hurt both you and your him- rearing, flipping over, bolting to escape the pain, etc. Even if your horse is not so sensitive in his mouth, seesawing will only harden it and then, to be effective, you will have to switch to a harsher bit. Then his mouth will harden to that one. And what are you going to do? Keep increasing the harshness of the bit until there's nothing sharper out there?
I know seesawing is common practice in the western world, but since the OP posted in the english riding forum, she should know that seesawing is heavily frowned upon. Not only is it harsh, it is not necessary if you know how to properly use your BODY (not just your hands) to ride.
My TB, Charlie, has a very sensitive mouth and his natural trot is very quick. When I want to slow his trot down, I control my posting. It is definitely an ab workout and takes a lot of core muscle to do so. But he will slow his trot to match my posting because otherwise his back gets thumped on and he wants to avoid that as much as possible. I count 1...2...1...2 over and over in the rhythm I need him to go. And that helps keep me in rhythm so he is able to get into a consistent, slower trot.
In english riding (correct english riding), riding a horse from back to front is key. Many disciplines focus solely on headset and don't include getting a horse to work from his hind end. The result from riding primarily with your hands is a horse that is on his forehand and is unbalanced. When you ride from your seat and your legs the horse must balance himself and will therefore develop the proper muscles in his hind end.