Although it sounds absolutely bonkers when you first hear it (and actually it sounds just as crazy even after you do some research on it), this actually works. And for good reason, too: The oral cavity and fluids in your skull do some kind of crazy wizardry to the remote signal when you're holding the key fob to your chin. It's kind of like the reason why those ancient "rabbit ear" TV antennas gave you better reception when you were touching them.
The New York Times asked a Silicon Valley radio engineer, Tim Pozar, for his take on how the trick turns your head into a DIY antenna.
Mr. Pozar explains, “You are capacitively coupling the fob to your head. With all the fluids in your head it ends up being a nice conductor. Not a great one, but it works.” Using your head can extend the key’s wireless range by a few car lengths.
If you ask William Van Winkle, it's a lot more than a few car lengths. He wrote this article for Tom's Guide earlier this month and totally confirmed that it increased his car's remote control effectiveness by about twice the length that it was before:
One step back beyond the point of being out of range, I confirmed that the remote didn’t work. Then I raised the remote, pressed it to my chin, and squeezed the button. I heard a “chirp!” in the distance and saw the headlights flash. Huh. So I started walking backwards. All in all I was able to back up 42 steps, roughly 85 feet, before I was no longer able to make contact with the car by holding the remote to my chin.