I tried them once...did only like 135lbs for 20 reps and literally almost blacked out. Never did them again but after a couple of more years of training under my belt maybe I will give them a try.

They are a great exercise, they tend to put more stress on the knees for me, but my back is given a rest.

how exactly are they done?

nevermind...i found it

If you can force yourself to drop your ass enough at the beginning of the movement, the shearing force on the knee is minimized. I was just tired of huge poundages compressing my spine on a regular basis with back squats. I'll probably end up alternating them in the near future.