Some dressage horses are very quick to alternate between coming above the bit and dropping behind the bit. If that sounds like your horse, you need to be quick to switch your aids as follows:
When your horse comes above the bit, use your “connecting aids”.
1. Close both legs to send him forward toward a lengthening.
2. When you feel the “surge” of power coming from behind, close your outside hand in a fist to capture, contain, and recycle that power back to the hind legs.
3. If your horse starts to bend his neck to the outside, vibrate the inside rein to keep his neck straight.
As soon as he ducks behind the bit, send him “forward through his body”.
Here’s what I mean by that. Go on a circle in rising trot, close both legs and ask for a trot lengthening for 6 or 7 strides. Do this several times until it becomes a knee jerk reaction for him to go “forward over the ground” when you close your calves.
Then close your legs as if you’re going to lengthen, but don’t let him lengthen. This time you want your horse to go “ through his body” rather than “forward over the ground”. As you feel him go forward though his body and start to take a contact with your hand rather than curling behind the bit, praise him.
You might have to alternate a trot lengthening with asking him to take a contact with your hand several times. But once he understands, you’ll have a tool to use when he curls behind the bit.
If he’s curled behind the bit really badly (not just going with the poll too low), in addition to sending him forward through his body, you might have to raise your hands to place the bit out in front of him so he can step toward it.
The feeling is like putting a sheet on your bed. You lift the sheet up, and then let if softly drift onto the bed.
You can also think of it like doing “the wave” at a football game.
If you do have to raise your hands because your horse has dropped behind the bit, keep the following things in mind:
1. Always use your legs BEFORE you raise your hands.
2. Raise both hands evenly.
3. To the degree that you raise your hands, ALSO put them forward toward his mouth without losing contact. That is, if you lift your hands 2 inches, they must go forward 2 inches. If you lift them 4 inches, they must go forward 4 inches.