Activate the Inside Leg

By activating the inside hind leg, the horse carries more weight on the inside hind leg and the hock becomes engaged and steps further under the horse, causing the horse to "sit" by lowering it's haunches.
Say you're on a twenty meter circle in trot on the right rein (though this exercise works well within any gate); to activate the inside hind: put your inside leg just behind the girth and give a "kick" to swing the haunches away from your leg. Congruently with the leg aids, keep a steady contact on the outside rein while lowering your inside elbow and bringing the right rein up and out to shift the horse's quarters away and bring the head to the inside. When done properly at first on the green horse, you can feel the horse "jump" away from your inside leg. Then straighten the horse and ride forward, and even allow a bit of stretch by giving the inside rein, thereby also checking the horse's self carriage. This is an excellent exercise for unlocking resistance.