Over-under cable coiling is a technique for winding up your cables and wires in a way that extends their longevity and reduces tangling, by working with the natural coil of the wire. It's easier to show than to explain, and this video from the London School of Sound does a great job of demonstrating the technique. Learn it, use it and detangle your life!
The "over/under" name refers to the practice of twisting the cable in one direction to make the first coil, and un-twisting it to make the next, and repeating this until all the cable is neatly coiled. Care needs to be taken to keep each end on its proper side of the roll when uncoiling otherwise a knot will appear with every other loop. Connecting the ends on the outside of the loops, or tying them in that position, insures that the ends don't pass through the loops in storage so there are no knots when the cable is laid out.
(via Wired Gadget Lab)