Also while the person is seated in the cart, have them lean forward and back. Stretch out their legs with spine against the seatback in a lounging position.
You the shaft holder will be AMAZED at the weight changes put on the harness by some shifting around. People get tired of sitting still, sitting in correct body position, so they do move about.
Then you will want to try holding shafts with TWO people in the vehicle. Again, moving by one or both people can have a lot of effect on the shaft weight on horse.
Horse has to “just DEAL WITH IT” a lot of the time. Part of being a Driving horse. He stays cheerful with changes that happen, keeps working, not reacting poorly because life is not perfect for anyone!
You can’t change how a cart shifts as you enter it or get down, gets shaft heavy on downhills with breeching pushing on him. People can not be expected to go driving and never move for an hour or more, unreasonable. Body parts go numb!! Helping the animal with a wide harness saddle, good padding under the saddle tree, will spread the load out. You try for good cart balance, a floating shaft most times, but sometimes float is just not possible and animal works a bit harder for a short time. You as passengers can sometimes make adjustments with your body or seat adjustment, to help horse for that short time.
Holding the shafts for people moving around in the seat is a real eye-opener on “cause and effect”. Some vehicles transfer EVERY movement, horse can feel you breathing!! Other vehicles have a more deadening effect with their shaft set-up, horse only feels big changes in body positions on the seat.
Do hold shafts TIGHT as you figure balance. VERY easy to loose control with two people in the seat, sitting wayyy back against the seatbacks. They can have more leverage than you might think!!