If you do a search, there has been a ton of discussion about overspeed/towing. The principle drawback of towing is that it pulls you into the ground so that your foot strikes farther ahead of the center of gravity, thereby requiring deflection at the knee for the hips to pass over, which predisposes the hamstring to greater risk of injury. It’s the exact opposite of what Charlie described as optimal technique wherein the hip is higher over the ground, thereby allowing the foot to strike closer to the center of gravity, thus reducing ground contact time. So in fact, overspeed training ingrains mechanics that will reduce stride frequency rather than increase it. This is not noticed when you’re actually doing the overspeed run because you have an external force added to your movement.
Adonail’s quote from Charlie above demonstrates how true overspeed can be achieved by a simple redistribution of effort without the need for gadgets and while preserving optimal technique.