a snap billet is like a snap hook on a lead rope or legstrap ( rather than a trigger hook that you have to depress a thumb catch to release). They allow you to change a bit very quickly so have some use in a schooling situation but the horse can easily get caught as it will snap onto anything that the horse rests its head on eg tie up ring/ haynet/ stable door etc. They were very fashionable with showjumpers about 5 years ago.