Free walk on a long rein is a movement in many Prelim and Novice tests.
It is included to test that the horse is relaxed but also, that he is working over his back and on the aids.
Often it is required on the diagonal line although is easier to ride when on the curved line. The rider should keep their hands low, near the wither and maintaining forward impulsion with the lower leg, allow the rein to lengthen by slipping through the fingers. Your horse should seek to stretch forward and down with his head and neck whilst still over tracking in the steps and staying on the contact.
Take the rein shorter again starting 2 strides before the next marker. It is important to keep the lower leg against your horse’s side and have relaxed wrists and elbows so that the horse does not anticipate the next transition.