Enough with big bad billy's Kandy Kid rave entry.
It is a flawed, unstable and useless technique. Not just because of the technique itself, but because there are so many better options available.
It won't be safe against a hard leg kicker, as it is both unstable and immobile at the same time.
It telegraphs what you're about to do, and as a direct result violates many of the fundamental principles of the system we all train in.
If the back foot never leaves the ground, you can't move much at all, let alone quickly.
In fact, your statement holds little validity when Big bad billy himself has released videos showing the technique.
http://www.youtube.com/watch?v=ZTXA5snEYa8
He clearly jumps up, and his rear foot leaves the ground.