The concept that I understand is that when the fish are just nibbling and barely inhaling a lure, the dropper hook rig tends to be easier to get sucked into their mouths. I've seen this effect and improved hook up ratio when I attached a secondary split ring chained instead of that single split ring. To improve it I've also seen better results while I chained a swivel in between both split rings. Then again when the bite's have gone to nibbles, I've just used fishing like direct to the spoon and then hook. I usually get results of within 1 inch to about 2 inches in line dropper length. However the concept for me isn't so much of the dropper rig, but it's to allow a better bite to hook up ratio. It follows the same concept of a stinger attachment and they are usually about two inches in length. When fish are just swiping at lure, the stinger has potential to get caught in their mouths.
While the dropper rig is more of using the spoon as a weight and/or attractant only, I've done so in faster current to that concept. This was where I used to just put a small tiny wet fly, micro jig, or single hook with micro plastics on. The problem I had was that during an up bite, well the weight of the spoon doesn't move. That two inches or more so is too much of a buffer of slack. By the time the fish has moved the spoon, they've already let go of the lure before I can react to a hook set.