A sissy squat is one of the most awkward forms of squats. We all know how squats are great for leg development. Regular squat targets your quads, but also glutes and even hams to a certain extent, making it an awesome, and the most complete lower body movement. So what's the problem with regular squats?
Bodybuilders who are looking to target only their quads don't like the fact that the squat activates the posterior chain muscles too - primarily glutes. That's where the sissy squat comes in - it almost completely eliminates glutes/hams, putting the weight strictly on quads. People also call it "bodyweight leg extension."
This old-school move was popularized by Vince Gironda, a bodybuilding legend, the guy who trained the first Mister Olympia, Larry Scott. The main reason why Vince preached the sissy squat is that he believed small hips are the key to that V-tapper. By eliminating glutes, lifters would be able to train their quads, making them bigger, without making their hips wider.