My dad brought one of these cups home from a baseball game for me to check out.
It is basically a plastic cup with a hole in the bottom. Around that hole, on the inside is a circular magnet. Then a magnetic disc covers the hole from the inside to keep the beer contained. The cup fills from the bottom up very quickly...delivering your favorite Blandweiser in record time.
Check out the video in the link. He fills 44 beers in one minute. I think it's a pretty crafty gimmick to speed up the lines at large events.