Examples of common positional mistakes -Heels down
This causes tension through the back of the calves and thighs into the pelvis of the rider
It tips your weight back in the saddle therefore putting pressure on the horses back and restricting their movement.
You can see this clearly in the video the rider posture becomes rigid and therefore becomes unstable creating a lot of movement.
If you have equal weight in your stirrups on the balls of your feet you will be able to sit tall and have you lower leg on the horse in order to ride correctly
You should have a straight line from Shoulder hip to heel.