Agreed. We have one technique (usually held until fairly late in the curriculum) that is used for this purpose. It's very simple, and takes a lot of skill to make useful (I had someone take me out with it during a kick - surprised the hell outta me).It is to knock an opponent off his feet (e.g. throwing, pulling, or shoving) To throw, pull or shove, you generally have to have your hands on someone. Your explanation is a trip. And in most circumstances, blocking feet will not cause someone to automatically fall down without the addition of a pull or a shove. Yes, there are circumstances were it may work, but I would hope that is not the basis of any martial art.