That's not a lesson you can build on to teach the horse anything else. They think if the consequences of a particular behavior are bad enough, the horse will avoid that behavior. But it doesn't work that way. The horse feels shanking, jerking, yelling, or popping as an attack. So instead of shaping the behavior the trainer really wants, these things just accelerate the behavior they were trying to correct.
Most people are scared when a horse rears up.
Their first reaction is to jerk on the lead rope or get out in front of the horse and pull on it. Pulling down on a horse's head gives the horse the feeling of being trapped. The fastest way to put a rearing horse over backwards is to keep pulling on his head because his natural tendency is to fight back against the pressure. Just the same, if you get out in front of a horse that's running back and start pulling on his head, the horse will just go backwards faster. You'll see horses running backwards with someone running right in front of them holding on to the rope and jerking.