Sounds like you're talking about "ankling." Sheldon has some knee-jerk (har har) anecdotal evidence about how bad it is in his glossary, but he's full of sht here.

There's also this, which is a little less full of hot air: http://www.faqs.org/faqs/bicycles-fa...ection-26.html

I tend to snap my foot down near the bottom of the downstroke to give it some momentum for the dead spot, then snap up with my calves on climbs or sprints, but that's me.