The dropper loop is a rigging knot, not a terminal knot: it puts a fixed loop in the middle of your line, standing out to the side, where you can attach a hook, a teaser or a second bait. It is the knot behind the high-low rig and any time you want to fish more than one offering.
When and why to use it
Use a dropper loop to build bottom rigs for scup, sea bass, haddock and fluke, to add a teaser above a jig or bucktail (deadly for fluke and stripers), and any time two hooks beat one. A loop that stands out stiffly from the line presents the bait better than one that lies flat.
How it works
You form a loop in the line, then pass the loop through the twists you create by wrapping the line around itself several times, and pull it tight so a firm loop stands out perpendicular to the standing line. Attach your hook to the loop (loop-to-loop or by passing the hook through).
Tip Take enough wraps and pull the loop out to the side firmly so it stands proud of the line. A dropper loop that flops down against the main line tangles and hides the bait; one that sticks out stiffly presents it and stays tangle-free.