It's designed to reduce/eliminate reflections glinting off the objective lens that may alert live targets to your presence. In that respect, it works as designed. Some reviewers have mentioned that the fit is loose on the 3.5x35 Trijicon scopes. On mine, it doesn't really rattle, and isn't loose enough to pop off on it's own unless there's rough contact with it. I noticed that turning/twisting it after popping it on tends to friction fit the tabs more snugly to eliminate movement. If desired, it's also an option to put a small strip of black electrical tape on the lip of the scope that the tabs snap onto to ensure a snug fit. The killFLASH does dim the view through the scope slightly, but I haven't noticed a huge detrimental effect, even when looking through the scope at night. While a screw-on option as on the SDO/MGO would be best, and the elastic retained option for the 4x32 might possibly be better, this works just fine for me.