Called Hiza Kakkun which translates to Knee... something - I'm not exactly sure. The point is to walk up behind someone, bend your knees enough to hit the back of their knees forcing them off balance. Yep, it's another crazy Japanese idea that you'll probably see kids do in America in another year or two.

I'm guessing it started out as a tripping method as a prank or for your own amusement much like how in Middle School we'd knick the heels off of the person walking in front of us. You can also turn it into a game like these Japanese girls demonstrate in the video below. Looks like fun

...if you like falling.