FuBarbell

The hips are a powerful generator when it comes to force production, but that by itself will not be enough to deliver the power in the right direction. For the productive application of force, learning how to pair the knees with the hips will get the bar moving up so you can get yourself moving down.

By @lesterhokw:

Producing Vertical Force.

It is definitely not wrong to say that the hips are producing force in the second pull. However, this extension of the hips is often mistaken as the sole proponent of increasing velocity of the bar.