This is actually a great question! The UTH is designed to be stuck the glass on the under side of the aquarium. The reason I choose not to install it this way is that this particular UTH gets hotter(115 degrees F) that I could feel comfortable for the snake to have pretty direct contact with. By covering the under side of the aquarium with foil and not placing the UTH directly to the glass it speads the heat out more and reduces the chance for a hot spot.
I just taped the UTH to the aluminum foil pad and didn't remove the film on the UTH.