Step 6: Rappel Down
All you have to do is squeeze the prussik knot, and it loosens and you can slide down the rope!
The downside is that you have to successfully do this to two knots at once;
fortunately, if you screw up, you just come to a halt on the rope.
You can also control your descent rate by how much you grip the knot,
which controls the friction between the rope and the knot.
I'm squeezing pretty hard, and making the accompanying face to show what it's like to loosen two four-turn prussiks enough to get it to slide. It worked well enough to prove the theory, but I abandoned the prussiks in favor of using an 8 for the rappelling.