That negative torque on the upstroke, where the back leg create a drag on the pedals, and the red and green lines on the bottom graph drop below zero, shows that not only are they not pulling the pedal up with their back leg at that point, they are actually semi-resting that leg, pulling up at less force than needed to lift the foot off the rising pedal, and instead letting the powerful front leg downstroke not only propel the bike, but assist in lifting the back leg.