Okay, I'm not going to let 190 get all the "cool credit" anymore
Here's some pictures of our spring loaded window motor disengager. We use it to lower our arm after a match. We still have to power the robot arm to get it in exactly the place we need but we use it to save on time a little bit as well as making it easier to carry to the pits.
http://i.imgur.com/O1NR1.jpg
We have the window motor coupled to a 3/8 hex shaft, which drives everything. The "seam" in the sprocket is where our makeshift dog connects to the sprocket in order to drive it.
http://i.imgur.com/2NsiC.jpg
Here clearly shows how the dog and sprocket connect, transferring power to the shaft. There's a hex pattern in the sprocket, but it rides on a round portion of shaft so the arm can free spin when disengaged.