It's the bit the horse works best with really. Snaffles slide through her moth and she throws her head around a lot. Not sure why she doesn't do it with a Tom Thumb, but we are thinking she was just started in that. I've tried a few milder, and she just acted differently (worse) and didn't perform as well because she was too busy messing with the bit. I've never tried those pictured. So I may look into those.
Not sure if Bosals are legal. I have used one in local competition not sure about higher.