You may have seen a similar video on my buddies blog, www.accelerateyourself.blogspot.com. We just started box squatting this week and decided to add some chains.
Box squat is a variation of the traditional squat where the lifter sits back onto a box, pausing at the bottom, then exploding upward. The keys to watch for on a box squat are to sit back onto the box, keep constant tenstion in the core so that the core does not fold over, and to explode straight up avoiding a foreward rocking motion. The chains add resistance as the bar and chains are lifted away from the ground.