Mr Curry is isolating the quads the best he can with this movement and probably better than most. Imagine the burn involved; anyone who experiments with this should find out right away.. Most BB'ers will just hold a plate high, in front of the chest when doing the original Sissy Squat. (I don't follow BB'ing, so haven't a clue who Mr. Curry is or was) Sissy squats can give an unbelievible stretch to the quads. Track athlete's use a form of this; warmup and general training. As do most other athletes.
That limbo move is a modified "S" curve. Front and regular squats pretty much follow the same pattern. Olympic lifters will usually squat from a rock bottom position, BB'ers..not so much. Olympic lifters have higher heels in their lifting shoes/boots, for better balance and a more solid postion. Very handy when squat cleaning 500+.
Some White Papers suggess less knee problems with full squats, than half squats (little below parrell). Than again, a few say the opposite. If having a potential for bad or weak knees, than raised heels squats, or even Sissy Squats, may not be for you. Your choice. Good Luck.