Back in high school track, Coach Wood would take us down to this extremely nasty hill maybe once a month. He'd send us up to the top and tell us to run down as fast as we could. This hill was steep, man. So, even with high-schoolers' levels of fecklessness and senses of invulnerability, we wouldn't exactly go all-out, because then you'd fall. At which point Coach Wood explained that it was only grass and wouldn't actually hurt, and that we needed to go so fast that falling was a risk, because the whole point of the workout was to run faster than we were capable of going under our own usual power, to get our legs turning over even faster than we thought possible. He did say there was one other way, which was to tie yourself to a really big, really fast runner, and then have them run in front of you and be forced to run at their speed to avoid being pulled over.
I mention this because I recently saw an ad on TV for the "Speedster," which appears to be a bungie cord with which you attach yourself to the other runner. They also showed clips of people doing football-style workouts that involved dragging heavy weights with it, but the emphasis seemed to be on leashing yourself to someone else and then getting pulled forward in pure high-speed running terror. Let me just say that the advertised price of $75 strikes me as a little high, given that you couldn't pay me to do a Speedster workout.