Ahead of the Motion

The term, “ahead of the motion,” refers to the rider’s upper body being too far forward, rather than being “with the motion,” or in balance with the horse. ¬†When the rider’s upper body leans ahead of the motion at take-off, the horse will be more inclined to stop at the fences, for the rider’s weight is overloading the forehand at a crucial point in the jumping effort.

The rider is "ahead of the motion," with her upper body leaning too far forward, rather than staying in balance over the horse. Her leg slipping back on the horse's ribcage is a large part of the problem. However, she is correctly looking in the air toward an upcoming fence on a tight turn, so her mind is working, although her position is weak.
This demonstration rider, Cat Montgomery, is "with the motion" of her horse. Her upper body is balanced over a well-positioned leg.