Scotty sells a snubber to install between the downrigger and ball. Its supposed to keep you from shock loading the downrigger line when lowering/stopping and snapping off the ball. Its also supposed to isolate the electrical field around the ball (if you buy into that kinda thing) I have never used one, but I could see it saving a novice from loosing a ball when learning to downrigger fish.
The bumpers are more important for electric downriggers. If they fail to stop (when retrieving) automatically, the bumper usually keeps you from snapping off a ball when it hits the downrigger pulley. I could see that the snubber might have much the same effect. I like to keep my downrigger gear as small as possible to minimize drag in the water, but I do use a bumper.... a large bead just barely big enough to not pass the pulley and a small piece of rubber tubing between the ball snap and bead to absorb some shock load if the bead hits the pulley
I use the scotty mini releases, and the larger one that quadfather was talking about. Simple and effective, but many dont like them. They just pinch the mainline like a clothes pin. I also use the diy "1 dollar kokanee releases" that the owner of this site (Mike Carey) did a youtube video about a few years ago. Theyre actually made from a plastic clothes pin, and honestly work just as well as many other releases. https://www.youtube.com/watch?v=0FNmPPmFM-8
link to the video