Sorry it took me so long to reply, Ronin.
You may want to knock someone away for a variety of reasons. It may be that you just want to "discourage" them.
The heel thrust does require the knee to be higher, since it works best as a descending kick. Targets are just above the groin or just inside the hip bone. Find out what 3 areas people get hernias in, and those are the spots!
This kick should also include that "hip roll" upwards some have described in front kicks.