The pedal stroke folks go at it again. The "only pushing down counts" folks always throw up the strawmen that (1) even the pros (2) don't pull up on the backstroke (3) when measured in a lab.

1) Pulling up on the backstroke is hardly the point of using either toeclips or clipless. Pros have been using one of these pedal systems rather than simple platforms ever since they were invented. If there were no advantage, they wouldn't use them. Toeclips always were a bit uncomfortable, not to mention dangerous.

2) Right, they don't normally pull up on the backstroke. This unarguably being the case, then what is the advantage if we dispense with that strawman? It's very simple and I think anyone who has ridden with clips or clipless for years gets it: The idea is to reduce peak force applied by any one muscle during the pedal stroke. This is done by spreading the load out among more muscles. People say, "Pedal circles," which does not mean "pull up on the backstroke." It means apply a constant torque to the bottom bracket. Those of us who use rollers know it as the commandment to "make the humm constant." IOW, don't be constantly accelerating and decelerating the bike with BB torque peaks. People talk about "pedaling squares" when they get tired. What they mean is that they lose the coordination to smoothly transfer effort from one muscle group to the next. We push forward at the top, put a little force on the downstroke, then pull back at the bottom and a little bit into the backstroke. Add the force vectors from both sides together to get a constant torque.

3) It's true that people being measured in the lab don't ordinarily pull up on the back stroke. However, they do on the road. Anyone who's ever won a hill sprint knows you have to pull up like crazy. I used to be the local group ride champ hill sprinter. I had to be careful to neither pull the back wheel off the ground and lose traction nor wheelie the front wheel off and lose steering. If one looks at video of great riders putting out max effort, efficiency be damned, yeah, they pull up. Watch LA vs. El Pirata on Ventoux:
http://www.youtube.com/watch?v=FXPXHK7I1iQ or http://www.youtube.com/watch?v=-Oql3qMNOqo
Watch their feet closely. I learned to pull up using the heel cups from watching Lance. Note that during their duel they don't always do it. Sometimes they pedal with their feet almost flat, but from time to time they engage their heel cups whether sitting or standing. They're trading back and forth quad for ham effort. The transition is quite marked. I also like watching the variation in Landis' pedaling action on the famous Stage 17:
http://www.youtube.com/watch?v=UHCRhzrSRA0
Sometimes he quite obviously ankles also, engaging his calves.