In the sitting trot Nancy has the rider squeeze with the knees to see how the horse reacts to that cue.  He slows to the walk, showing how tightening can change the trot, and how sensitive the horse is to tightening by the rider.  They go off the lunge line and apply the awareness gained by being on the lunge.  They repeat the progression to eventually sitting the trot all the way around the circle , making sure the trot doesn't get to big.