Double knee bend

The "double knee bend" is what occurs once the first pull ceases. It is a "repositioning" of the body, which initiates the powerful "explosive" phase in Olympic Weightlifting. It also occurs during a properly executed vertical jump. It is a vital component of proper vertical force production.

Hip Position As It Relates To The "Double Knee Bend"
Starting with the hips too high or raising the hips faster than the shoulders during the first pull, will create a horizontal position of the torso relative to the platform as the bar reaches the knee. A horizontal torso at this stage of the lift causes excessive torque at the hip. The greater the distance between the bar and the hip, the greater the torque. This excessive distance between the hips and the bar decreases the effectiveness of the body's powerful extensor muscles. It also increases the distance the torso must travel in order to get vertical, which forces an elongated and slow "double knee bend." These technical errors cause a significant decrease in bar velocity (rate of change of position of the bar) during the "double knee bend" and should be corrected.

No "Double Knee Bend" At All
There are some that say there shouldn't be any "double knee bend" at all- that one should go from extending the legs in the first pull directly to back extension without a repositioning of the knees. The reason given for this type of pull is "repositioning of the knees decreases the velocity of the bar. While it is true, there will be a slight decrease in velocity due to the repositioning of the hips and knees, it is important to note, the quadriceps will be close to fully contracted at the end of the first pull and can no longer contribute to force production. The "repositioning" allows the lifter to re-utilize the quadriceps once more to produce the necessary upward force at the end of the pull. The slight drop in velocity seen during a properly executed "double knee bend" is an accepted component of efficient Weightlifting technique and does not negatively effect the outcome of the Snatch or Clean.

Here is a brief description of what is actually going on during the "double knee bend."
Once the knees finish extending during the first pull (at a knee angle of approximately 145° in the Snatch and 150° in the Clean) the lifter will extend the torso upward and backward bringing the torso to an angle of approximately 60° relative to the platform, while forming a hip angle of approximately 110° in the Snatch and 115° in the Clean. The knees and hips move forward and down forming a knee angle of approximately 120° for the Snatch and 125° for the Clean. The bar moves up to the middle of the thigh for the Snatch and the lower third of the thigh for the Clean. The weight will shift slightly forward towards the middle of the foot (more so in the Clean), with the shoulders slightly ahead of the bar. This entire action should take 0.1 to 0.2 seconds to complete.

These actions put the lifter in the most effective position for using the powerful hip and leg extensors during the 2nd pull (the explosive phase of the Olympic lifts).