Basically the air that comes in to the rad wont be able to escape up through that gab directly behind the grille forcing more air to hit it. We arent talking big temp drops (to pull a number out of my ass, prob average is 6 or so), but every degree counts....and i was bored.
Yeah. Like i said i just did this with pretty much my hands, a vice and tin snips lol. Better tools would def wield a cleaner result.