I believe they are describing a different motion where your ankle is flexed upward (heel up).

When I practice "wiping mud off my shoes" I extend my heel down as the pedal comes through the top of the arc and keep it down (so that if my foot were to contact the ground it would contact first), and then rotate through to a more "ankle neutral", straight position through the upstroke.

As my equestrian trainer always used to shout "LONGER STRONGER". (ie: make your thigh and calf as long as possible by dropping your heel and you'll get more power (ok, on a horse it was stability and cue strength, but concept is similar)).