Yes, that's the context in which I've seen it as well. I've seen it in young (foals, some yearlings) horses and they do it to say "but I'm only just a baby" to a dominant horse
Ironically, I just watched a PBS special about wild horses in which the foals did that very teeth clicking thing to the older horses as a sign of submission. It was so neat because I had just read your post.
Exactly--Jazz did it to the mares a lot when he joined us as a foal, but I noticed Claire, who as a weanling thought she was the alpha mare, never did it to anyone.
Trust me, you'll get to know that "teeth nashing" motion more and more....... It's gotten to a point when we see it we speak for them....."I'm a baby! I'm a baby!"
Yup, I've seen that with most the little ones around here. My stallion sometimes runs in an arena next to a bunch of yearling colts, and every time they touch noses all the babies do the teeth clicking thing. My gelding Charm did it when he was first meeting the mares he lives with now. Since he was one boy raised by 3 moody girls, he's pretty well gotten the picture that he's always at the bottom of the pecking order.
......so now even though he's a bigger stronger 2 year old, he still does it to every horse he meets!! Horses are funny.