It is good practice for the observer or driver to acknowledge hand signals by repeating the signal back to the skier. But if the skier has not signaled, then it is essential that the driver give the skier the appropriate signal before changing speed, direction, etc. This gives the skier warning and allows the skier to adjust their ski until the maneuver is completed.Theres one more signal Ive seen used occasionally, which I think skiers may use to express disapproval of the drivers abilities or perhaps behaviour of others in the boat.