It's crazy how effective this is. My coworkers when I was on this kick at a restaurant used to make a game out of trying to throw me off balance, often with a drink tray in hand. Literally charging at me. Didn't drop any glasses.

For 15 minutes of working out a day.

It won't make you look like a bodybuilder, but if that sort of core strength is what you're after it very much does work.