1. Judges should take care not to mistake hyperextension of the knee (the angle at the back of the knee exceeds 180 degrees) for failure to comply with the rules. In these examples, both athletes land with a generally straight leg but the knee angle continues to change as the leg passes under the body. Observing an athlete taking three or more steps per second can give the illusion that the leg is only becoming straight after heel strike.
2. Watching the landing forces on both the shoe and the leg indicates how much braking force is acting when the foot lands far ahead of the body's center of mass. Keep your step in front of the body shorter than the step behind your body.
As the great coach Frank Alongii assured us, "You are a rear-wheel drive animal." Frank worked on the lunar rover design and tended to talk in automotive analogies.