I'm not sure what you mean by shank most gag bit's I've seen have been on snaffles. I can sort of see what you're saying about how it moves before fully engaging, but I don't fully understand how it could be more mild than snaffle with fixed rings, not shanks.
Here's the way I see a gag bit working, but please correct me if I'm wrong.
Pressure on both reins pulls the rope through the bit, this applies pressure to the horse's poll, the bit slide up pulling the horse's lips up, potentially hitting the horse's teeth (but I'm not sure if that could happen or not), at the same time it tilts the bit down toward the horse's tongue. Depending which type of mouthpiece your using.
I imagine the poll pressure is what helps keep a horse's had down when doing cross country or jumping at good speeds. I imagine that the slide in the bit doesn't so much cause a gagging action as the name implies but it would certainly increase the intensity of the pull. then the tilt of the bit would push pressure onto the horses tongue.
Now that's all my assumption from messing around with one in my hands, I don't use it and have only seen it on a few jumping horses but never really paid attention. But I DID notice that each horse I saw that used a gag bit also needed a flash or some other nose band that ties their mouth shut, which I found interesting, but that's just the ones I know.
I'm also well aware how a person rides in the bit greatly changes the bit, so I guess I'm looking to find out whether my assumption about the bit is correct or not? If I'm wrong please tell me how it really works?
What do you mean snaffle? I do not consider any gag bit a snaffle, regardless of mouthpiece or look of the shank. The gag itself creates a shank, therefore leverage, therefore not a snaffle.
Yes, it is true gag bits apply a lot more poll pressure. They are meant to encourage dropping of the ehad into a stop and turn type of fashion, which is why they are popular with barrel racers. However, the contact needed on the mouth is significantly less.
I am a barrel racer mainly and I use a lot of gag bits. If anything, it quiets the horse a little more IME. The horse will generally gape their mouth due to pressure put on the mouth, but since the majority of the pressure is on the poll and there is the gag allowing softer mouth contact, there generally isn't a huge amount of it.
That is how I have been operating anyway.
My original post here had a significant amount of gag bits on it. Including combos, mild gags, and bits with more gag. You of course have to understand how the bit works, and how to use your hands with it, but I do not consider them harsh.