The clip on adapter is a reflector that enhances the signal to the antenna, and as such it does not need a direct metal to metal contact to work. However, that increase in signal has to come from somewhere, and it comes from a narrower angle that the antenna will accept. Instead of a nice wide cone of acceptance, it amplifies (focuses) a narrow portion of it. Aiming is more critical.
I had a unique situation that pointed out the strength of the design. I was parked in a small town without a TV station, and the closest ones were 70 miles to the north and 45 miles to the south. I could not see any signal from either before putting on the add on director gadget, and was able to pull in digital signals after installing it. The direction you point the antenna is very critical, because of the narrower acceptance angle.
Radio engineers have been using this technique for years, and I picked it up working with HAM radio.
As mentioned it is very easy to install (you would have to modify it and really work at it to get it on wrong), works well using established antenna physics (not a "magic" black box no one can explain), is inexpensive and works. The main thing you give up with it is the antenna has to be more carefully aligned in the direction of the signal, and that is a one time deal until you move again.