Double knee bend

The transition from the first to second pull in the snatch and clean and jerk is arguably the most important transition for generating power in the Olympic lifts. Most elite lifters rebend the knees at this transition (referred to as the scoop or double-knee bend), which allows for better power transfer from the hips and legs. The figure below shows the double-knee bend in two lifters (panel 4); once the bar passes the knee, the torso straightens and the knees rebend slightly to bring the barbell back over the knees and feet.This rebending generates the characteristic S-shaped trajectory of the barbell:This rebending is very clear when looking at the angular velocity of the knee joint in the figure below (which also illustrates the rarely seen split-snatch).Coach Burgener highlighting the transition from the first pull to the second:

The first and third figures above come from this very nice paper. The second comes from Tom Gorman's site, which is absolutely packed with tons of amazing information on Olympic lifting. You can track down more information at these links (1,2,3), although keep this in mind.

My favorite example of how this transfers to sports can be found here.