One of the more subtle nuances of working from the boxes is remembering to initiate with a leg drive before starting any other part of the second pull. Most of the block work that I program is designed to be completed at below the knee and therefore it is absolutely essential for the precise execution of the lift that you initiate by driving your feet into the floor. Even from the top of the knee this is a necessity. Starting the lift with a leg drive will force the knees to get back and stay back allowing the lifter to be over the bar with their shoulders.
This also ensures that the lifter is able to create more velocity with their legs before creating it with their back. Everything has to happen in sequence and a long, sustain leg drive must come long before the use of the back.
When working from the blocks remember to think about pressing the floor away from you to ensure that you utilize your legs as the primary source for power. This will inevitably prevent the lifter from engaging the back early and rotating backwards into the second pull too early. This is one of the rampant bad habits in the Weightlifting and Crossfit world that often will send the bar out in front of the lifter and looping around their head. If the lifter can sustain a leg drive for as long as possible it allows the lifter to move vertically with the bar rather than move the hips horizontally into the bar. Vertical hip motion creates upward bar motion while the horizontal hip motion will send the bar away from the lifter.
Below are two really good examples and analysis of two very different lifters performing that leg drive very well off the boxes.
Side view clean from boxes leg drive emphasis from Power And Grace Performance on Vimeo.
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45 degree angle clean from boxes leg drive emphasis from Power And Grace Performance on Vimeo.