For example a "box" exercise. Ride a straight line (start at jog), stop and back a few steps, pivot 90 degrees, trot off in striaght line, stop, back, pivot, go. This encourages them to use their hind end.
Later you can apply this at the lope as well, and even ask for the sqaure corner without stopping, simply lifting the shoulders.
Lots of transitions, stopping and backing throughout your ride. Try not to make it predictable all the time, this gets your horse thinking.
As well ride a circle, stop turn 180 on the haunches and go the other direction on the circle, when they start dragging, stop pivot 180 and go the other direction, continue to use this till your horse is moving correctly around the circle.
For more information like this you should access the people on
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