A flash is something that connects to the front of the bridle and I guess it keeps your horses mouth closed and stops them from chomping. It doesn't hurt them if you're wondering
You could try using a bit with copper rollers this softens his mouth and let's him play with the rollers rather than the whole bit
He is an old trot race horse so i am kind of scared to use a non-joined mouthpiece since it was what he used in races, but I'll buy one and try!
What is a flash? :P
Tried a non-joined mouthpiece?
If he chops a lot, try using a flash on your bridle.