Here’s an exercise to supple your horse’s poll.
Start in the halt on the rail so you can check that you’re keeping your horse’s body absolutely straight. If he’s straight, his body is parallel to the rail from nose to tail.
When you start to supple the poll, keep his neck parallel to the rail. The most common mistake is to bend the neck. Your horse can bend his neck and still stay locked in his poll.
Use an indirect rein to move his face only one inch to the left and one inch to the right so you can just see his inside or outside eye and/or nostril (this is also sometimes called position left and position right, flexion and counter-flexion, or +1 and -1).
Remember, when you use an indirect rein, keep your fingers softly closed around the reins. Then, turn your wrist as if you’re locking or unlocking a door, turning the ignition key (right hand) to start your car, or scooping a spoonful of sugar out of a bowl.
Don’t vibrate the reins while suppling the poll. That will just flex your horse’s jaw and close the angle at his throatlatch.
When turning your wrist, keep your hands stay side by side. In the moment that you turn your wrist, your fingernails face upward, your baby finger points diagonally up toward your opposite shoulder, and your hand comes quite close to the withers.
Once you’ve turned your wrist, return to your “starting position” with your thumb the highest point of your hand. That is, don’t hold your hand in the position with your fingernails facing up and your hand near the withers.
Never bring your hand across the withers. Also, be sure you support with the opposite rein so your horse doesn’t just bend his neck.
If you’re next to the rail, you’ll easily be able to see if you’ve used your opposite rein enough. If you haven’t supported with the opposite rein, your horse’s neck won’t be absolutely parallel to the wall anymore.