Yes Pat -as Peter indicated that is a "Pro-Release down rigger clip". It's purpose is the same as the Scotty release clips I showed you on my boat - e.g. you attach your fishing line to the clip and send it down.
The clip you have appears to be set up to be attached directly to the down rigger ball as opposed to clipped onto the cable like I do. I prefer to clip them onto the cable a little above the down rigger ball as then you can bring the ball up to just below the surface and get at the clip. Doing this will keep the ball from swinging all over and hitting the boat if you're in any waves.
This
link at Salmon University shows you how to use the clip. I have a few of these but have quit using them and reverted back to the Scotty clips. There are two reasons for this.
First, the pro-release clips don't release by pulling on the line at the rod end. That is, you can't pop the line out of the release just by lifting up hard on the rod. Hence, if you want to check for weeds etc. you have to bring the down rigger up and reel in at the same time, unclip the line by hand etc. This is difficult to do with a manual downrigger and even with an electric down rigger adds another step (e.g. you have to touch the clip twice - once to get the line out and once to put it back in). With the Scotty clips, you simply give a good tug on the line to pop it out of the clip, reel in, check for bait or weeds and then bring up the down rigger ball.
Second, the Pro-Release clips are difficult for newbies to use and if you wrap the line around the post in the wrong direction they won't release. That is the line has to be wrapped such that the end toward the fish comes off the end of the post first. If the line is wrapped in the opposite direction or if the line crosses over the top of another wrap, it can bind up when you get a big fish on and break right off. I lost a couple of nice fish the first few times I used that type of clip due to this problem. I can make the clips work reliably now, but there is a skill to it and the Scotty clips are easier and more reliable in this regard. The nice thing about the Pro-Release clips is that it is easy to adjust the tension on them. That is you can adjust them to release for even very small fish, However, that the only thing they have going for them.
I'd also recommed that you buy a
rubber snubber to put between the end of the cable and the downrigger ball. This will reduce the wear on the cable but more importantly, it gives you a nice thing to grab to lift the down rigger ball out of the water (it doesn't cut through your hand like the cable does when you grab it).
For connecting the ball to the cable, you will need a terminal clip of the type others have shown you. They're easy to attach and if you buy the
Scotty ones, they come with all the hardware necessary to connect them. Buy two or three and buy 2-3 down rigger balls. In the winter, when we fish for local "black mouth" (resident or winter chinook salmon), you fish very near the bottom and occasionally you will lose a down rigger ball. The only thing more frustrating than getting out for a day of fishing and losing the downrigger ball in the the first 10 mins of the day is doing the same with no replacement parts on board.
I would also recommend that you buy or set up down rigger release clips with about 6' of 130# mono filament line on them so that you can reach out with a dehooker or boat hook to pull the clip aboard without having to lean out to grab it. Gary Johnson taught me that trick and it makes things a lot safer - especially if fishing alone. Finally, I'd be happy to go with you to Outdoor Emporium or some other fishing gear shop and help you find what you need (read "spend your money"). I had a friend do this with me about 5 years ago and it was very helpful as he showed me most of the gear I use today in one 30-45 min session at the store.
