If a bit asphyxiates the horse then they would drop dead wouldn’t they? They wouldn’t be able to be ridden either as they would collapse from a lack of oxygen. A horse isn’t in the cross ties when you put the bridle on. They are out of the cross ties with a halter on which you then slip the bridle over and remove the halter. Then you place the bridle gently on the horse’s head and the bit in the horse’s mouth. If the bit asphyxiates the horse we would all have to buy a horse every five minutes!