Okay the ring feature on the karambit is about retention. With a karambit a blade is attached to the handle. That blade is hard to grab and apply leverage to break the finger. Now, I am not saying it is impossible to leverage a karambit and break the holders finger just that it would be hard to do in a very dynamic situation. There are disarm techniques though that do this. The edged advantage of the karambit allows you to cut someone if they are grabbing your knife bearing hand provided you know what to do. Even if a karambit bearing hand is slashed or the person utilizing loses a finger or two they can still function with it as long as the finger in the ring is not cut off. You can even form a master grip on a handgun because of the size of a knife with most karambits and I have absolutely no problem functioning with a shotgun or long gun with a karambit in my off hand. This makes the karambit a very effective and efficient knife.
I think this is more about someone trying to come up with some thing that looks cool/fancy and has that shtick to sell to the masses. Hey, we invented the karambit tactical flashlight. Yay us! I do not think you need a retention ring on a tactical flashlight and because it is not a blade and you cannot cut your way out when interdicted. I don't see it as valuable. Just another schtick in a long line of someone trying to sell some thing. Not to mention that it would be possible to get your finger broken if your opponent gained control of the flashlight somehow.
No one disagrees with the advantage a tactical flashlight brings as a blinding agent and or a force multiplier. That is not what this conversation is about. I know that I simply would not be without one. Whether working or not. I can remember one instance when my family and I were at the Grand Canyon and after having a fancy meal at one of there restaurants we went outside and it was pitch black. Fortunately I had my light and we were able to avoid going off the path and falling to our deaths. My kid's were young at this time. The advantage of having a light is monumental in any low light situation.
We can also look at the usage of the ring/tactical flashlight in question and think about having to transition to another tool/weapon such as a long gun and the time if might take to get the ring feature off quickly to do so. It is not much fun to operate a long gun with a tactical flashlight in your off hand. Very, very challenging to do so if not just plain stupid. Your long gun/shot gun should have a mounted light. Much easier just to drop a tactical flashlight without the ring feature and transition to your long gun with a mounted light feature. I would also look at the use with a handgun you could certainly use the FBI method of use with a flashlight, you could also use Neck Index technique and the Harries Technique. You could not however use the Surefire technique to form the master grip on your firearm. If you wanted a solid master grip on your handgun you simply could not do it with this ring feature tactical flashlight due to the cylindrical nature of a tactical flashlight. You would again have to discard it which would take some more time to do so than if you had your non-ring feature tactical flashlight.
Tactical Flashlight Firearm
Techniques
Handgun Master Grip
Possible broken finger if opponent gains control, slowed down in transitions to any other tool, Cannot grip long gun, shotgun or handgun with a master grip without getting rid of it. I'm just not seeing the advantage of this ring feature!