How does a zip line work?

A zip line works by way of gravity and inertia. A cable stretches across a gap, be it a canyon, or a gully. A harness is attached by "lifeline" to a pully that slides the expanse of the cable it is attached to. A person in the harness simplylifts legs off off the ground to soar down the cable. Built momentum from the "flight" down, carries said person up the other side of the cable. That's a zip line!