Take a look at 9.4.3 Situation M in the 2018 casebook. Particularly option (c) and (d). In C, the tackler rides A1 for several yards, before A1 falls backward. Foul for horsecollar because the runner fell backward. in (d), same scenario, but the runner fell forward. No foul for horsecollar. Like I mentioned earlier, whether you call this a foul or not depends on your interpretation of forward and/or backward. If backward means the direction from whence he came, then this is a not a foul, because, even though the runner fell onto his back, he fell forward. In fact, we would mark him down at the point where his FORWARD progress ended...