Such positioning challenges the driver with controlling the horse's motion. The driver has to trust the horse. She called this contact a datum, or a benchmark amount of contact. The driver never drops rein contact, as the reins form lines of communication with the horse. The horse must learn to tolerate the feel of harness, shafts, and traces.
Many horses also resent the crupper's being fitted under the tail.
Comparing the difficulty of driving with riding, Moore explained how a ridden horse carries only a finite weight, the total of human and tack. Driving can be more difficult because the conditions make the vehicle the horse pulls vary greatly in its difficulty of pulling it. You can have only so heavy a rider, where the steepness of a hill and the depth of mud can increase to infinity if the conditions are bad enough. It can make a horse that can pull a light vehicle down the road with no trouble at all unable to pull the vehicle at all if the mud is deep enough and the hill steep enough. He has to pull himself and the weight of his load.