This is the body position at parallel in a normal squat:
This is the body position at paralell of the Perfect Squat:
In case it is not clear arms are against the sides in the perfect squat but they are extended forwards in the conventional bodyweight squat Another difference is that legs are shoulder width or wider in the conventional squat and they are together for the perfect squat).
The perfect squat is not actually possible, it would require infinite force to sustain. But as you move from the normal squat to the Perfect Squat you will be in effect applying greater and greater force. The only limiting factor is your strength.
When trying this maintain a lot of body tension and go slowly. This will allow you to find the limits of your strength without tipping over.
The principle at work here is torque and you can make similar modifications to make other exercises insanely hard. It definitely works for pull-ups, or lever or planche variations.