Rappel tip - Getting the knot past an obstacle

Scenario:  You’re the last one rapping on two ropes tied together and you’re worried about the knot sticking on the corner of the ledge or in a crack.  Try this trick to move the knot past the obstacle: 

Girth hitch a two foot single runner *above* the knot, clip the other end to your belay loop with an extra biner (not the big locker to which your rap device is attached), and begin to rappel. The knot will move along with you as you rap down.  Once the knot is past any possible ledges or crevices where it might hang up, unweight the rope for a moment, unclip the runner from your harness, remove it from the rope, and continue your rap to the bottom.

A few cautions:
1) Practice this a few times in a controlled environment.  It’s a little unnerving to see one side of the rope NOT sliding through your rap device!
2) Note that doing this will cause one end of the rope to rise, so be sure both ends of the rope touch the ground or reach the next rap station.
3) This trick is a little more complex on a free hanging rappel, as this make unweighting the rope a slightly harder maneuver.
4) Be sure and use a short single runner.  Using a longer runner may cause the knot to be out of reach when you want to remove it from the rope.