A lot of riders aren’t clear on the difference between flexion at the jaw and flexion at the poll, and how to ask for each of these positions.
Keep in mind that a horse can flex three ways–to the left, to the right, and “in”.
When a horse flexes to the left or right, he’s flexing at the poll. When he does this, you’ll just see his inside or outside eye or nostril. I call this position +1 or -1 because you’re bringing his head 1 inch to the inside or the outside of where his head would be positioned if his chin was directly in front of the crease in the middle of his chest.
You’ll ask for flexion at the poll to the left or right with an indirect rein aid. To give an indirect rein aid, give a quick turn of the wrist so your thumb points to the center of the circle, your fingernails point up toward your face, and your baby finger points up toward your opposite shoulder. As you turn your wrist this way, bring your hand very close to the withers, but don’t cross over them. As soon as you’ve turned your wrist, return to the “starting position”where your thumb is the highest point of the hand. Be sure you support with your outside rein as you do this so you “catch” his poll rather than bend his entire neck.
When a horse flexes “in”, he flexes at the jaw, and he closes the angle at his throatlatch. You’ll use a completely different rein action to ask your horse to flex “in” than the action you used to ask for flexion to the left or right.
Ask your horse to flex “in” by moving the bit in his mouth. Be sure you only use ONE rein to move the bit. If you alternately saw on his mouth with your left and right hands, he’ll just bring his face closer to his chest.
We often flex the horse’s jaw. In fact, his jaw must be flexed for him to be completely on the bit. BUT, the danger lies in flexing the jaw BEFORE you connect him over his back. If you flex his jaw first, he’s not really connected. His face is just “in”. The danger here is that you can fake yourself out.
You might think he’s correctly on the bit because he feels soft in your hand when his jaw is flexed. But if you go to do something like a transition, you’ll find out that he’s really not connected at all.
During the transition, he’ll raise his head and neck and look hollow because all you have control over is a flexed jaw. He wasn’t honestly on the bit to begin with! You need to ride your horse from back to front. Close your legs and send your horse forward through your outside hand to get his back round. And ONLY after you’ve sent him forward through your outside hand should you flex his jaw as the final ingredient of putting him on the bit.