Think of it as a magnifying glass for your scope. It doesn't change the image or field of view, just the size of the image as you see it. Everything is bigger so gives the impression of more mag.
What I like about it is I don't have that tunnel vision you get from high mag scopes. It didn't change the brightness or contrast for my eye. Magnified the image - that's it. Resolution is just as good as the orig scope.
What I don't like is the loss of eye relief. Don't use it on a hard kicker with a low eye relief scope. On my F class rifle and 223, it works great. A scope with at least 3.5" would be ideal (better if closer to 4").
For the money, I really like mine.
One warning is that it changes the occular focus. Most scopes can have the eyepiece change so that reticle focus is restored. I tried it on my new Sightron SIII LR and the reticle never did get sharp. The image quality was superb but the reticle was just a bit fuzzy. No biggie but may bug some shooters.
It is not something you can put on and off as the scope focus has to be adjusted with and without.
Jerry