Were you down at the archery range when your scoutmaster showed you how to tie a this handy knot? Wikihow shows you the moves.
The sheepshank is a useful knot for two purposes. It can be used to temporarily shorten a rope or it could be used to allow a rope with a damaged or frayed part to be still used (the damaged part would be in the center, where there is no tension). One of the properties of this knot is that it can get easily undone without tension, so to keep it in place, tension needs to be applied on both sides of the knot.