As far as the definition of "parallel," it's defined as when the crease of the hip is in line with the top of the knee. Remember, most people have very poor hamstring and hip strength to squat properly in the first place. If they tried to squat without the box they'd fall over backward. The box is the best way to teach proper squat form while bringing up their weak points. The box squat also breaks the eccentric/concentric chain. This is one of the best ways to build explosive strength. The box squat also causes you to squat from a static contraction to a dynamic concentric contraction, another very effective way to build explosive strength.
When you reach the box you want to sit down and relax the hips flexors while keeping every muscle other muscle tight. You also don't want to fall down on the box and try to bounce off of it. You sit back with the same speed you squat. Pause on the box for a split second and explode off of it. No bouncing! Your knees must still be pushed out and your abs, upper back and arms should remain tight while your back stays arched. When you're on the box it's important to have the shins perpendicular to the floor or better yet, past perpendicular. This places all the tension on the squatting muscles.
Phase IV: After you pause on the box you need to explode off by first driving the head and upper back into the bar, then by driving with the hips. When you begin the squat (during the eccentric phase) the hips move first then the head. The opposite of that (the concentric phase) should involve the head moving first then the glutes. It only makes sence to try to lift the bar first. If you don't drive with the upper back first then the bar will begin to move forward. If the bar is moving forward before you drive with the hips, you'll miss the weight and fall forward.
As you're coming up you still need to maintain all tightness by driving your back into the bar, driving you head into the bar, pushing out on your knees and feet, pulling the elbows forward, keeping the shoulder blades together, and holding your air. After that there's nothing else to do but lock out and wait for the crowd to cheer.
That's all there is to it. And they say squatting isn't a technical lift! Now it's up to you. Do you want to be standing there watching others lift the big weights, or on the platform doing it yourself? You decide.
If you'd like to get more info from Dave Tate you can contact him at Elite Fitness Systems at 888-854-8806 or
EliteFTS@email.msn.com.
For more info on his seminars, check out the following link:
Deepsquatter.com/efs/service.htm