How does a bit go into a horse's mouth?

Usually a bit can mean the horse is not well behaved and needs a firm tug to the mouth.The bit does not hurt the horse unless the horse resists what the rider wants.For example: If the rider wants the horse to halt and the horse shakes it head and keeps walking the rider will have to pull on the bit until it hurts the horses mouth.This will only happen about 2 times and the horse will then understand that disobedience only causes the horse pain.