For lunging the use of one inside side rein will encourage the classical rule of having a bend around the inside leg that will create flexion and encourage lightness. One side rein keeps it simple and less restricting therefore the horse will not be put in a fixed position. Lunging correctly is an exercise to encourage a round position, on the bit, and forward movement. Lunging in a low relaxed position will stretch the back muscles and encourage relaxation.
Trey, an Arabian lunging low to encourage relaxation