Originally Posted by Brian DeGennaro
The idea of keeping your feet planted (they will move as necessary) will ensure more force is transferred to the bar.
It's nice to keep your feet as close to the floor as possible, but in my experience they won't move as necessary. Often the attempt to keep them planted means cutting the extension short, rather than simply not picking up the feet so much. A brief moment of zero contact with the floor as you transition to pulling under the bar guarantees zero resistance, which means more potential speed in that most critical time.