Ride the exercise by travelling down the long side, turning the corner as if to make a circle, but instead, angling back to the rail. Go straight for a stride or two at the edge of the arena and then angle back out and circle around towards your starting point. Ideally, you want to make both loops the same size.
This is a great exercise for improving rider precision in bending and straightening your horse. It also prompts your horse to ‘listen’ rather than assume he has to make a full circle.
To begin with, you should keep it simple by staying at a walk and trot, but over time you can try it in the canter, changing canter leads on the straight line in the centre.