This simple gymnastic is helpful to tune up your horse and slow down his mind. I find that placing a ground rail on the take-off and landing side of a vertical, followed by an an oxer gets the horse to slow down a bit at the fence and really use his body. This is one of my go-to exercise for between-show schooling sessions.
This exercise isn't about the fence height, it's about getting them to slow down. When the vertical is 3-foot-3 to 3-foot-6 (or even lower), placing the rail nine feet in front of, and 10 feet behind, the jump is a good distance. When using these ground poles with an oxer or with a larger fence, I push the pole on the landing side a bit further out. If your horse is not used to the exercise, give them an extra foot or two on either side of the jump.