basically they open their mouth and cross their jaw over...
you can see it and you can feel it, they quite often chuck their head in the air and run off a bit as the bit is basically not doing anything any more...
Its a favourite of my boys, although he doesn't do it too often (mainly just in halt) when he does it he becomes very heavy in my hands and will not flex
R will cross his jaw and try to piss off without a flash - so I ride him without a flash for flatwork because I can circle and stop in the school, but with when out hacking otherwise I could get in trouble.
Mine did it badly- we tried a grackle and a flash, just made things worse... i then went with no noseband for a month or so and then went back to just a noseband- problem pretty much solved!
Personally I keep the tack the same and when I feel the horses do it I squeeze left and right until it has released the jaw. if that does not work I slow down (half halt/ a transition within the pace) and try again!
I prefer to know I can solve a situation myself and think I have to get off and rely on a change of tack for a very simple problem! I think it is much safer that way and makes you a better rider!
Just personal opinion, if it is combined with strong shoulders/ a set neck/ running off I would try a flash.
My mare crosses her jaw and has strong shoulders/tries to set her neck against me. It's a case of keeping flexing her to stop the neck from setting, and gently giving and taking the contact to get her to relax her jaw and soften/relax.
I find a drop noseband is much more effective than a flash.
Crossing the jaw is when top & bottom jaws go in different directions. EG horse moves top teeth (jaw) to left & bottom teeth (jaw) right. Actually I've just tried this on myself & only my bottom jaw moves, my top jaw stays in the same place all the time but my bottom jaw can move right or left. If you cross your own jaw you'll understand what it means.