It creates a loop that can be shortened and lengthened easily at the knot and holds the length when tension is applied. Attach the loop to a point in the ceiling, clip a biner to your bag, clip the loop on, lift the bag by pulling the standing end and making the loop shorter.
You can also use a Midshipman's Hitch:
http://www.animatedknots.com/midshipmans/
Thank you. I'll have to try it. I have a ratchet system but I'd like to see this work.