Here's something I think can be helpful...let the other folks with more experience say if I'm wrong.
Get a friend to hold their hand out flat, with the bit resting on it as if it is the horse's tongue. Try to keep it at the same angle that your horse's head normally is at. They can also lightly hold where the bridle attaches. Attach the reins, and then pull at the angle they would be at while riding. Pull one, and pull both. Watch how the bit folds, moves, etc.
If you have two friends who don't mind your geekiness, let one position the bit in between their two flat hands, simulating the mouth. The second holds where the bridle attaches, and now you pull the reins.
Try walking along your horse, and seeing how the bit functions as you ask for turns and stops. You can't see inside their mouth, but you can gain some knowledge of how it affects them.
For today's ride, I switched from this one (as shown in post 17):
Back to this one, only using the small ring below the big ring:
It functions more like a normal snaffle, with some poll pressure added. I had stopped using it after a ride because it is more of a squeezing action, and it didn't make sense to me to cue in two opposite directions at once.
However, I took my mare out of the road using the bit from post 17. When she felt more excited, and didn't want to stop when asked, I pulled back enough to engage the curb strap - and she started flinging her head, nose above her ears and thrashing from side to side. I took her back to the arena and we did some work on "no head thrashing"...which seemed to work, but I decided curb straps and my mare would need a lot more arena time. She had NEVER flung her head upward, and I didn't want her to start.
She behaved fine on the road today with the elevator bit. If the squeezing action confused her, she hid it well.
For now, I'll switch back and forth depending on what we're going to do. I'd like Mia to learn to ride with a curb bit, but I may have been pushing her too far beyond what I had prepared her to do.
I just wanted to share that because A) horses should always have a vote, and B) I think it is up to the rider to TEACH a new bit rather than just assume one will work.