Lunging is a way to teach forward movement which is necessary for lightness.  You can also very carefully use a side rein to accustom the horse to contact.  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.


Lusitano stallion in Brazil.


Paladin, a Friesin cross