Some horses do well in a gag. There is no requirement to use a gag harshly. I only used one for a couple of rides. My horse didn't care for it so I stopped using it - but it didn't cause her to raise her head, hollow her back, etc. Those symptoms come, not from the bit, but from a rider ho doesn't give a release.
It happens with any bit that is poorly used.
On a journal thread, an experienced rider I respect has picked a gag for a horse who gets excited on runs, who can be a challenge to control when excited, but who dislikes curb bits. I reject the idea that all horses must learn on "mild bits only" because an excited horse may not have enough control of his emotions to obey a mild bit, and won't LEARN that control unless ridden, in the open, while excited, with a bit the horse WILL respect. Once the horse LEARNS the RIDER has ideas worth listening to, and how to control his (her) emotions, the horse can start listening in a milder bit.
Mia was a good example. In the arena, you could ride her with your pinkie on a snaffle. She didn't get super-excited when in an arena. Out in the open? That was a different question, and she knew how to resist a snaffle. So I switched her to a curb bit. 3 rides in the arena to teach her what a curb bit felt like and how to get her own release with it. Then I took her out on the trail. That first ride out she panicked about a motorcycle. She tried to bolt, the curb bit was enough to restrain (curb) her...and moments later, she realized the motorcycle was now running AWAY from her!
I could almost see the light come on above her head. "You mean, I can stand still and the bad thing will go away?
" I couldn't guess how many bolts she had done, but there was only one more time she thought of bolting. That was when she scrapped against a cholla cactus with her hind leg. She leaped forward. I got her to stop in 3-4 strides....then dismounted and pulled all the spines out of her leg. That was the last time she tried to bolt. She'd still spin violently when startled, but she never tried to run away with me again.
She wasn't going to learn to stand her ground in a snaffle. She knew HOW to fight it and, by fighting it, it made things even more scary in her memory. A curb bit taught her a valuable lesson. She always liked curb bits more than snaffles, although I rode her in both. Eventually traded her for Bandit, and Mia became a brood mare who is now mostly ridden in a bosal.
Point being that excitement and/or fear - the horse has a hard time distinguishing between the two - can get a horse amped up. Think of it as background noise. It is easy to listen to someone when standing next to a mountain lake. Much harder in a noisy bar. Excitement is like a noisy bar to the horse's mind. You may need to shout to be heard - but once the horse learns to listen when excited, you won't need to shout any longer.
Mia in the arena:
Mia a couple of years ago on the Navajo Nation with a couple of her babies:
Harsher bits are not harsher if the horse resists a "milder" bit. Gags are basically a snaffle bit - except the linear pull of the snaffle is amplified mechanically. A curb bit rotates, applying pressure against the bars and tongue regardless of head position, and can be useful with a horse who has learned to resist the snaffle. But a curb bit can be a gentler bit, MILDER, than a snaffle once the horse has the self-control to listen. Curbs and gags are not just for finished horses. They can be good tools to get a horse past a problem.
PS: A horse can have an elevated head and a relaxed back. A lot depends on the horse's build and personality. Bandit ALWAYS has a raised head:
Trooper rarely elevates his head. Difference in build and personality: