David Torrence demonstrates high knees here. High knees are used to increase leg turnovers and improve knee lift. This is a great drill for increasing strength in your hip flexors. Upright form should be maintained the entire time.
David Torrence shows the fast leg cycle. He takes a half step in between each running motion replication, but he is isolating each leg (alternatively during the drill) to emphasize form and control over each. Knees should always be high with heels kicking towards the butt.