Rising on the correct diagonal on rising trot

In rising trot, you either ride on the left or right diagonal. To chec whether you're on the correct diagonal, look at the position of the horse's shoulder blade. When the shoulder blade of the outside of the arena is at its most forward position, you should be sitting in the saddle. When the shoulder swings back, you should be out of the saddle.

This rider is rising on the correct diagonal.
source from http://www.crystalclearequestrian.webs.com/