It works by cutting a slot on the spine on the very top of the mag so the bolt catch leg protrudes into the magazine to make contact with the top of follower after the last round is expended.
A shooting colleague turned me onto this idea. It took me a few seconds to click in what he meant as it's one of those simple ideas that'll have you think why you didn't think of it yourself.
He mentioned some people thought he was a nut job and that it would not work with an Uzi mag. Probably thought it was too simple to wrap their head around the idea, that if this worked, someone else would have though of it already. I guess someone else did. Goes to prove old guys know stuff.