Laurens has work on collection with a lengthening in the frame and we see a profound change in the balance.  In the trot the horse shows a lot of suspension and expression, but they want to be able to collect with a quicker rhythm.  He asks that the rider bend him in preparation for the half pass, but keep his neck up and his frame  in an uphill tendency.  When they school the half pass they have to make sure he doesn't become too extravagant in his movement and that he keeps the collection.