One of the most common mistakes in canter is that of the rider 'polishing the saddle' with their seat. It has been told to many riders as an analogy for sitting to canter. This is incorrect - elite riders keep their seat in the same place in the saddle for the whole of the canter stride. One of the side affects of this incorrect way of sitting is that it causes the rider to be heavily behind the vertical in the phase of canter in which the horse is most on the forehand. Watch the video below which is part of the full lecture.