To make a knot "slippery" ie: slippery half hitch, slippery sheet bend, slippery whatever, simply form a bight with one of the working ends and tie the knot with that. Like tying your shoes, you don't just work the the straight end of the lace, you fold it in half and tie the knot with those two "loops" aka bights, and when the knot is complete you are able to simply pull on the end of the lace and it comes undone. The difference with your average slippery knot is that you only need one bight in it to make it slippery. (not my image but like this)