a chifney should be used on a horse that rears because when the horse rears the bit flips up in his mouth and hits the roof of the mouth to make him associate rearing with pain. If your horse doesn't rear there is no need
I don't have a lot of experience with chifney bits, and most of my research into the specific uses of it took place in the last couple of days
, so this is, as always, my humble opinion.
According to my research, a chifney's common use is just as you described, to control a known rearer, or for extra oomph dealing with a stallion. Going by your description of its action, put yourself in the horse's shoes: You're a) scared. That cave on wheels is no place to be. You want out. 2 People are coming at you from behind, trying to push you in, there's a swinging door blocking your escape to the right, and the person leading on your left won't let you turn that way. The only escape left, aside from sacrificing life and limb in the horse eating box is to go up, to rear. You go up, and the roof of your mouth hurts, what would be your first instinct? Go higher, get away from that pain that comes up from inside your mouth. Yes, I suppose that over time if the pain consistently comes with the rearing, you may learn that one leads to another, but initially, throwing pain into an already stressful situation for the horse can only aggravate things.
Scenario b) the horse is confused/not understanding. Picture yourself in a math class, oh, say, calculus. You don't understand the material, what to do with this slew of numbers and letters. You ask for help, and the teacher comes over and smacks a wooden ruler hard over your knuckles. Did you learn anything about calculus? Nope. You just learned that asking the teacher was not a way to go. Same goes for the horse and the trailer. He didn't learn anything about getting on, he only learned, at best
, that rearing was not the answer.
Scenario c) "The Bullhead." The horse is not afraid, he has loaded before quite successfully, he understands what is expected of him, he simply refuses to load. It isn't his idea right now. The problem lies in a lack of leadership of the handler, and a lack of respect from the horse, pure and simple. A horse does not respect someone they do not follow as a leader, he does not follow as a leader someone he does not respect. The two go hand in hoof. Inflicting pain does not make you a leader; instilling trust in your decision and confidence into the fearful horse, taking the time to correctly teach the confused horse the way to "solve the problem," so to speak, are ways to become a leader, to gain respect.
The horse gets on the trailer because he (scenario a) respects your judgment and trusts that you will not put him in danger, you are a good leader, (scenario b) you have taught him things before, if he thinks and responds accordingly to your requests
to what I term as forcing, literally taking away all other options, I.e. Winching the horse onto the trailer with a tractor and pulley, doping the horse until he's barely making the descision, let alone learning, etc. Thses are not requests.) life will be good, or (scenario c) you are a leader worthy of respect, and the horse shall deign to get on the trailer because he respects your request and your (unknown to him, granted) plans for the immediate future.
Maybe there are horses out there who are such dangerous rearers that pain is required to conrol the animal at all, but such an animal is unlikely to be going to a competition, as is the OP's horse. As always, my humble opinions.