The famous Vince Gironda Sissy Squat. In summary it is a front squat with the addition of pushing your hips outwards in front of you with your knees over your feet. People have mistaken the roman chair squat for the sissy squat - when they are nothing alike.
The sissy squat as an exercise takes on the form of a leg extension/squat more or less. You may want to start without any weight, and just holding onto the smith bar for balance. Your body should be straight from the knees to the shoulders and you simply lean back as far as you can, and then come up.
The next 5 reps are from that lowest point you reach when you are doing the first 5 (so lean all the way back), and then fall to your ankles as if you were doing a full squat. From that full-squat position you don't stand up, but rather, thrust the hips forward to get back into that low position. Then back down to your ankles again.
The final 5 reps are the combination of both. Lean back as if you are doing a limbo. When you reach that low position fall down to your ankles, then thrust the hips forward to come into that low "limbo" position, and then come up. This explanation above might need a bit of coaching but the other alternative is to take a look at the video above and see a demonstration.