My horse opens her mouth when I ride her. It does not seem to have a pattern, but then again I don't have video footage of me riding that I can watch. I assumed that she gaped when I got too handsy, but I had my trainer watch for it as we rode and my horse opened her mouth when I had fairly light contact and wasn't really messing with her mouth at all. She doesn't open her mouth for long or very wide.
The first thing I ruled out was myself. I try to be more aware of my hands when I am riding so that I do not fall into the bad habit of pulling on her without releasing. Of course this is a work in progress, but I like to think that I am slowly improving in that aspect.
The second thing I checked was her teeth. The vet came out to float her, but after he took a look, he said that they don't need to be done yet.
My horse will often try to weasel her way out of doing hard work and only do it half-way and thus do the work incorrectly. I have to correct her and get her going properly. I think that she gapes to avoid the contact and thus "escapes" from working correctly.
Under the reccomendation of my trainer, I ride her in a flash. I experimented with it and found that if I had to too loose, my horse would use the flash as leverage to pull on me and go around very heavy in my hands. When I adjusted the flash snugly, she worked better. When I rode without it, she was almost as good as that, but I could tell that she somtimes was evading the bit and fighting it a little.
I ride her in a double jointed loose ring snaffle. I'm pretty sure it's stainless steel. It's like a knockoff of the KK Ultra Sprenger bits, so it has that same rotated middle piece. It's more on the thick side. The only other bits I've ridden her in is the actual KK Ultra Sprenger bit and a french link. They're all similar bits and she was the same in each of them.
Let me know if you need to know anything more. Oh my horse and I mostly do dressage.
Although I seem to have found my solution, I'd like to know if there are other options that I have not considered yet. I also know that a lot of people really dislike the use flash nosebands, so I wanted to know what they would do in this situation as I am stumped. I want to do the right thing for my horse's comfort and training. Thank you!