When an enemy is behind you, hold back+crouch and press attack (using Medium, Strong or Dual Saber style).
You'll do a "two handed" backslash that has a greater chance of doing a headshot than a normal backslash.
You can do an odd twist on this move with Staff (someone on the forums suggested it first, I forget whom). Switch to Medium style and then do the move, as you start the move immediately turn the other blade back on (!).
It doesn't seem to help you much, but it looks kinda cool.