The Elastic Training Rein works by putting gentle pressure on the poll encouraging the horse to soften. If the horse raises it's head from the correct position the poll pressure will be increased to encourage the horse to drop its head.
Start with the aids as loose as possible until the horse is comfortable. The aid should then be shortened in small increments. The horse should be working comfortably for several sessions before the aid is shortened again to allow the muscles to develop correctly and gradually.
improves overall outline
improves suppleness and therefore improved paces