Complex sentence: Modern competitors are starting a trend to a more classical position, with the leg placed underneath their body and their hips over their heel, a position more forgiving on the horse's back that encourages better movement.
Split into simpler sentences: Modern competitors are starting a trend to a more classical position, with the leg placed underneath their body and their hips over their heel. This position is much more forgiving on the horse's back and actually encourages better movement.