The basic description of the exercise is transitions on progressively smaller circles. The best benefit happens with trot-canter transitions, but also can be done with any kind of combination of gaits like walk-halt, trot-walk, canter-walk or for advanced horses trot-passage.

• Start on 20 m circle in working trot rising or sitting, trot the circle until your horse feels rhythmical, swinging, pushing from behind to the bit, equal in the reins and bends nicely around inside leg.
• Ask for the canter transition without losing line, frame, balance/connection. Canter once 20 m circle and come back to trot. Make sure you come back to the same trot you had before canter. If it feels faster, stiffer or a horse starts falling in/out fix the issues before advancing to the next step
• In trot spiral in to 15 m circle, pay attention that the energy and swing remains in the trot and bend becomes more pronounced.
• Trot a circle or two and pick up canter again. The circle stays 15 m and the canter should feel a bit more slower and more contained Make a transition to trot, same rules apply as for 20 m circle transitions
• Again, in trot spiral in to 10 m circle. Now the horse may start losing energy, bend or balance. If it is possible to improve the trot within 2 circles you can canter. If not spiral out to 15 m to pick up canter
• On 10 m circle pick up canter, canter about half a circle and start spiraling out all the way to 20 m. Spiral out steadily with same tempo and balance, the horse should not speed up or fall out into outside shoulder. Remember, spiral out means you remain on a circle during spiraling. That requires constant attention to steering! No part of the horse should get ahead of the other.
• Variation of this exercise is spiraling into a smaller circle before making a downward transition. This can be surprisingly more challenging then the above version!

Troubleshooting: Transitions are the most vulnerable moments during our rides. Any part of what makes a gait nice can be lost during transition: line, connection/frame, engagement, balance, swing, etc. A lot of riders "blank out" to make a transition - meaning - they stop doing what they were doing before to make a transition. This exercise reveals such mistakes. If during a transition to canter the circle size or frame of the horse changes a rider most likely dropped contact, giving reins away in order to let the horse go into canter. If connection was correct, contact felt nice and elastic no extra giving of the reins necessary.
Another common mistake is stiffening the hip either for upward or downward transition. Stiff hips do not tell the horse what gait is required of him. Thinking of canter or trot rhythm for corresponding transitions can be very helpful to avoid stiffness and giving your horse valuable cue.
Don't be impatient and kick or pull your horse into a new gait. For upward transition a tap of the whip after normal aid can be very helpful to get his "attention". For downward transition, give your horse time to find coordination to change canter from trot and not lose his balance.

Happy riding...