The knot starts out like a clove hitch. One turn around the spar and over the standing end and back over the spar. You can do this either to the left or the right.
Take the bitter end and pass under the spar and over the first completed turn. (A clove hitch here would go under that first completed turn.)
Then take the bitter end and pass it under the start of the first turn.
Finally take both the standing end and bitter end and snug the knot tight. The knot can be made for quick release by placing a loop in the bitter end before placing under the start of the standing end.
I use this for securing items to railing or life lines. Even fenders secured this way on small life lines will not slip. Quick, easy and secure.
Finally, below are some variations of this hitch.