If your fit is good, you should be able to pedal either heel up, or heel down.

heels up uses the calves, you will feel the burn there.

heels down uses the quads and glutes. you will feel the burn there.

the ability to make that adjustment on the run, giving one or the other a chance to recover without coming off the power, is very useful.

You can easily demonstrate the effect for yourself by going up your stairs on tip toes, and then doing it again with heels hanging off the steps.

not sure why everyone seems to think this is some kind of binary thing with a religious experience... but then again, it is bikeforums.