What follows reveals two secrets of advanced knot tying. Tying bight on bight is key to rapid knot tying. Secondly, how ropes are held in the opening step of knot tying is vitally important.
The double fisherman's knot is used as a handle and wound around the climbing rope. This step also positions this bulky knot out of the way.
Note that the effort is made to form the top three wraps or coils. They're held in an organized fashion. The winding below the double fisherman's knot is allowed to flow down the rope. Effectively these wraps are ignored during this stage. They become organized in the next step.
Here's the key configuration that aligns everything. It's formed by pulling the two bights of rope in opposite directions. For example, hold the climbing rope bight in your left hand and the loop in your right hand. Then pull your hands apart. The Prusik coil which wraps both climbing strands is called the bridge. Keep it positioned at the bottom during your pull.
When you straighten your climbing rope, the Prusik snaps into place. This all happens in less than 10 seconds once the steps are practiced.
This Youtube video will help you: http://www.youtube.com/watch?v=6TuGDsoi-Oo