This is a great exercise I picked up from a dressage clinic I took in high school. While I have never been a dressage rider or owned a dressage saddle, I have truly come to realize the value of really great flat-work.
You can do this at any gait to work on transitions however it’s meant to be done at the canter. Begin halted in-between the poles. Prepare your horse and make sure he/she is properly set up to pick up the left lead canter. The goal is to get your canter directly from the halt. Complete your circle and return in-between the poles at a halt. Then repeat to the right. Continue until you feel you and your horse have made progressed with leads and transitions.