I've described this drill before. It's not all that unusual to find yourself fighting with a rapier in one hand and nothing in the other, and the purpose of this drill is to help you defend yourself in this situation, especially if you've been unfortunate enough to have your opponent trap your blade so you're left with nothing but your empty hand to defend yourself.
As I noted before, these are Jeet Kune Do parries applied to fencing. The actual hand motion that you use may vary.
Note that when I miss a parry, Ysabel stabs me in the throat. That's how a drill should work: if you don't execute your maneuver properly, you take a hit. The person making the thrusts should be on target every time. Incidentally, that's also why you need to have full gear to do this drill.
More to come...