This overspeed training device is the safest overspeed trainer you will find. By attaching this device to 2 runners you can overspeed the runner in back with comfort and security. The back runner allows the front runner to stretch the elastic up to 3 times its length. This then allows the back runner to be pulled forward with the help of the elastic. 20' safety sleeve elastic with 2 swivel belts and stretches to 60'.