Is that really a thing? Who knew? Over the weekend, I was flipping through the Mylers' book, The Level Best for Your Horse, when I stumbled on an article in the appendix. As a side note, I've said this oh, at least 47 times already, but this book is totally worth having in your equine reference library. You can get it here. Anyway, the article was written by Dr. Joyce Harman, DVM, MRCVS and is entitled "Anatomy and Physiology of the Mouth as it Relates to Bits."
Dr. Harman goes on to explain that there are other muscles that connect the hyoid bones to the jaw and poll. The jaw houses an important nerve center for proprioception, that thing that tells a horse where his feet are without him needing to look at them. We have the same system - we know where our limbs are without needing to see them. It's how we do things like a drive a car. Tension in the tongue affects that nerve center which affects a horse's coordination system.
What all of this means is that when a horse's tongue is free and soft, he will move much more freely with better coordination and balance. Izzy is the first horse I've owned with such a sensitive tongue. With that said, it's difficult to move to a bit with minimal tongue pressure as tongue pressure helps the rider keep control. As Izzy gets more and more broke though, I am seeing him move better as I transition him into bits with less tongue pressure.
I am really excited to keep trying out the low extra wide ported barrel bit!