It's something they do with horses in cowboy country, Annie. The horse is taught that when the rein is on the ground it is fixed there; eventually just dropping the rein to the ground is sufficient to 'tie' the horse in place.
The technique is written up in Kelly Marks' Perfect Manners; if I knew where my copy of that book was I could look it up and tell you a bit more!
The way the boy carefully placed the handle of the lead on the ground, and the way the dog stood and how it behaved while the lead was on the ground just looked to me as though the dog was 'under control' rather than making its own decisions.