Rolling Hitch (video)
The Rolling Hitch is used in sailing to relieve the tension on a line or chain. You can slide it in one direction but not in the other.
To tie the knot make a turn around the rope/chain, crossing over the standing line. Then make another turn, following the first turn (you can make extra turns if you wish). Finally to complete the knot make a half hitch just like the clove hitch.
Video demonstration rolling hitch