The first of a series of videos by Dale Mylar is below the picture of Mia. Good discussion, but my horses almost NEVER have their heads held vertical, nor do I want them to. Mine normally is at the angle below:
The red line goes from the bit to above the horn where my hands are, so the pull is on the reins is nearly parallel to the her jaw - a very different pull than what Dale illustrates. I think this shows how the horse carries herself for a given use can affect the bit's action. The angle of the front of the D-ring makes it easy to see the angle of the pull on her mouth. And normally the pull involves taking slack out of the rein and maybe my pinkie's weight as well...so it isn't as though the bit will fold noticeably. My GUESS is the main effect is some pressure on one side of her tongue.
I would expect a very different action for a horse ridden in a different style (random internet photo):
However, I like's Dale Mylar's discussion of the tongue and how it affects bits and how the horse feels the bit.
In any case, it is a good excuse to think about bits...