The box squat can correctly be done one of two ways. One way you just use the box as an idication you are reaching a particular depth. Using this method you squat down until you touch the box and come back up. Doing this frequently will help you reach the correct depth when squatting without the box.
The other method is to sit on the box, relax the hip extenders briefly, then push back up with all the speed you can. This method builds power coming out of the hole as well as teaching you how to "sit back" into the squat. Using this method you can vary the box hieghts to target different specific points in the strength curve.
The motion begins just like a normal squat. Notice the placement of the box.
Here is the trainee making contact with the box. Notice it has him right
on the legal depth mark. You can vary this height to target different points
in the strength curve.