Your horse should be able to flex laterally at his poll to the left and right. If you're not sure if he's locked at the poll, ask yourself some questions:
Will he easily flex to the left or right with one quick turn of your wrist or does he stiffen against the action of the rein?
Does he tip his head on small circles or lateral work with a bend like shoulder-in?
Are his ears level when you ride either to the left or to the right?
If he stiffens against your hand or tilts his head when you ask him to flex left or right, you probably need to supple his poll.
Here's a 2-part "poll suppling" exercise to help your horse.
Part 1. 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 will stay parallel to the rail. The most common mistake is to bend the neck. Your horse can bend his neck and still stay locked in the poll.
Think of moving his face only one inch to the left and one inch to the right so you can just see his inside eye and/or nostril (this is also called position left and position right or flexion and counter-flexion).
Do this by keeping your fingers softly closed around the reins and turning 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. That will just flex his 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 and your baby finger points diagonally toward your opposite hip. 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.
Don't bring your hand across the withers. Also, be sure you support with the opposite rein so he doesn't just bend his neck. If you're next to the rail, you'll easily be able to see if you haven't supported enough with your opposite rein, because your horse's neck won't be absolutely parallel to the wall anymore.
Part 2. Once it feels easy to get position left and position right, pick one of those positions, and put your hand forward toward your horse's mouth to put a little loop in the rein. If you've suppled your horse's poll successfully, he'll stay flexed in that direction and not "boing" back with his face in the other direction. For example, flex him left, give the left rein, and see if he stays flexed left without your hand.
Once you can do this at the halt, go to the walk. When you can do it in the walk both to the right and to the left (flexion and counter-flexion), ask in the trot. Once you can get the answer you want in the trot, go to canter. Don't expect to get anything in a faster gait that you couldn't get at a slower gait. Also, if you have success in the trot, but not in the canter, go back to the trot (or walk or even halt) until you can do the second part of the 2-part exercise successfully.
A Happy Horse