If you do front squats deep like that, chances are you are getting a good stretch position stimulation. Sissy squats are difficult to describe. You really have to see them performed to know how to do them correctly. But, I'll try to describe 'em anyway:
-stand between the uprights of a squat rack and hold on to the rack at about waist level with a reverse grip (hands out to your side, palms facing behind you).
-bend at the knees, but keep your thighs in a straight line with your upper body. This will force your upper body to lean back as your legs bend...remember, keep your upper body and thighs in a straight line.
-As you bend the knees, you'll need to keep your balance by holding the uprights of the squat rack
-As you lower your knees toward the floor, your heels will come off the floor and you'll be on the balls of your feet.
-As you lower your knees, your upper body and thighs will be at about a 45 degree angle with the floor, and your heels will eventually touch your hamstrings at the bottom of the motion.
-DON'T STAY AT THE BOTTOM OF THIS POSITION, as the quads are totally stretched and there is alot of pressure on the knees too. Do these in a piston-like motion where you hit the bottom and immediately come back up.
You won't need any weight to do these. Bodyweight works just fine.
You might get a trainer at your gym to perform this so you can see what it looks like.