Good Question! I saw them on a popular saturday afternoon fishing show, but the big retailer that sponsors the show don't carry them and I could not find them anywhere - what a waste! So I had them professionally made.
The photos below should show you, but they are a means to lowering the rod tip to the water so the lures you are trolling are not likely to pop out of the water because the rod tip is pointing to the sky which is often the case in a standard rod holder. Outriggers are another option but I have never priced them. I shudder to think!
Here it is with the rod placed into it.
The tubes are quite specialized - here is one reason - the "gimbal" at the bottom of the tube is designed for the slots in the bottom of your game rod, in my case the wilson live fibre.
There also has to be a gimbal slot done at the bottom of the tube so that when a big fish hits it, the tube does not spin in the direction of the fish. When I had the tubes made I also took the original rod holder inserts into the guys to re-do the bottom weld in the rod holder. The original "tack" welds looked dodgy and I didn't trust the strength of them. If that spins for any reason, say goodbye to your rod and reel which will immediately point to where the fish is, and the rod would definitely come out of the tube. I still tie a rope to the nearest cleat and also to the reel.
Last photo - they also put a rubber insert into the tube - to protect the rod from any damage while in the tube
Hope this is helpful.
Cheers
Scalem