Ted Blocker, the inventor and creator, has great videos on his website that demonstrate how this thing works. The quick version is that if a horse jerks back, the rope will slide through the tongue of the device. This release ALWAYS satisfies the horse's need for escape, and he stops pulling, usually before he pulls all of the line through.
It's also an excellent tool for trailering. In the event that a horse falls in the trailer, his weight alone is enough to pull enough slack through the tongue of the ring to prevent him from choking or being "hung up." Depending on how the rope is fed through the tongue, the amount of "pull" needed for the release can be set. Since Speedy never "tries" to wiggle himself free, I use it on the loosest setting possible.