When you are in midstride, the arm that protrudes is opposite to the leading leg. The protruding arm is on the same side as lagging leg, which should be in midair.
As depicted, the the thigh slips into one cuff, and the forearm/wrist slips into the other. Guided by a few plastic rings on a specially
designed shirt, a semi-stiff, bungee-ish cord goes up the lats, around the shoulder and finally down to the hamstring. It stays on one side of the body.
When one forearm protrudes, the bungee cord pulls the back of the hamstring up and back, which should allow the arms to contribute to the forward motion. Because the forearm pull starts while the lagging foot is still on the ground, the cord will aid the glutes in pulling the leg backwards. (You can also see in the diagram that the left arm is bent behind the body. As it shoots forward, it, too, will pull the left leg back, while the left leg is on the ground.) One must adjust his or her arm swing in order to maximize efficiency.
The ring on the back may have to extend out a bit further than is shown on the diagram, in order to provide more horizontal force.
The diagram shows the right leg after it is FINISHED exertion, and is ready to move forward again. The red arrows indicate that the bungee HAS moved, (and NOT that it is going to move), in the direction depicted. (A device that pulls your back leg over your head wouldn't help your running.)