A gag bit includes poll pressure. There is some leverage, but what I notice is how Mia responds to the additional pressure behind the ears. For riding, I use the reins connected to the small hole below the big one. When I pull, it brings the mouthpiece back (like a snaffle), but the leverage squeezes the entire bridle, so it also pulls the bridle more tightly down on her head, creating pressure at the poll.
What Mia would do when excited is stretch her head out nearly horizontal. If she then clenched her teeth shut, all I could do with a snaffle is pull it against her molars. When she was excited, she knew she could ignore any pressure there - the old term is 'having the bit in your teeth', as a synonym for being uncontrollable.
With a gag, if she does that, she can ignore the pressure against her molars, but the squeezing action puts pressure on her poll, and she has no way to ignore that.
We have since moved to her current bit.
The sides act independently for the first 45 deg of movement, so I can normally treat it a lot like a snaffle and direct rein. However, the bar in her mouth doesn't fold, and if I use both reins at once, it becomes a standard, one-piece curb bit. Since it has leverage, it needs a lighter hand on the reins. The first few times she became upset if the curb strap engaged, but now she knows what it is and she can prevent it from engaging by simply behaving. It also uses poll pressure.
I suspect this bit CAN hurt her some, but she gets to choose what level of pressure to obey. She can respond when I take some of the slack out of the reins, or when I gently pull the first 30 deg of movement on the lever, or when I give a couple of tugs on the reins, or when I pull it hard enough for it to squeeze and for the curb strap to engage. She doesn't like that last option, so she is getting good at responding at lower levels. When she starts to get nervous, a flick of my wrist on the reins is enough to remind her I'm there and that I don't want her to run...and she seems to be getting the idea. She also seems to be getting the idea that when I do that, standing still makes the bad thing go away just as effectively as running - so why not stand still?
That isn't needed for a lot of horses. Trooper does just fine in a full cheek snaffle. But Mia has always been nervous. Some of that has improved with training, but the right bit IS a training device. If I can 'force' her to stand still, then she can learn that standing still makes the bad thing go away without effort, and that her rider will let her know when it is time to run.
And she carries it in her mouth in a more relaxed manner. She isn't head tossing, or constantly tugging on the bit the way she did with most snaffles. In every snaffle I tried with her, sooner or later, she would try to take control of the bit. She would drop her head way down, then snap it up. Or she would pull constantly on the bit, and we would return with my shoulders aching - and I weight 180 to her 900. Or she would go quietly for 10-15 minutes, but start head tossing when she felt nervous.
Most of that is gone now. Over the last few weeks, she hasn't done "The OMG Crouch" once. She is moving down the trail more relaxed, without the tension in her back. If she gets too nervous, a few flicks of my wrist will get her to stop, and then I can rub her withers and neck. If she prances, another tug and she will stop, and I can scratch her neck some more. By that time, the scary thing usually has moved away. Or she is calm enough that I can back her, help her calm more, and then move forward.
I do not in any way want to suggest it is a good bit for every horse. Each horse is different. But a bit is no more harsh than a crop, IF you always go thru the progression. If you always use a crop to get a horse to go faster, it never learns. If you always start with a kiss, then a light leg squeeze, then a bump, then a light heel tap, then a firm tap, then a kick, and then a crop, sooner or later, the horse will figure out that moving faster at a firm tap is better than waiting for the crop. And eventually the horse will figure out that moving faster at a kiss is better than waiting for a firm tap.
Same with the bit. The gentlest bit is one your horse can carry comfortably in its mouth and that the horse also respects. After that, it is up to the rider to teach the horse that since you will stop anyways, you might as well stop the moment I take slack out of the reins, and ultimately that you might as well stop when you feel my seat settle.
But if your horse doesn't respect the bit, then it becomes a harsh bit. I rode bitless for my first 3 years of riding, and I could and sometimes did remove hair from Mia's face trying to control her. What is gentle about that? And with a snaffle, I sometimes needed to darn near tear her head off to get her not to bolt, or to stop in a bolt - and what is gentle about that? When she wasn't excited, she has always been able and willing to obey a sidepull halter. But it is what they do when excited that determines what bit they need. After that, it is up to the rider to train them, just like you train a horse to soften to 'go faster' cues - by always using the same progression, always insisting on eventual obedience, and letting the horse figure out that the easy way is more fun.
All just IMHO. I've been riding for 5 years, but I don't compete or teach or have perfect horses, so take it with a big steaming cup of FWIW!