Head up, like a "crunch." Heel off the ground, six inches. Works the lower abs and obliques.
When a coach catches you slacking or snoozing, he'll step on on your lower belly. Instinctively, you flex your lower abs, thus sharpening your focus and muscle work.
Trust me, if we had Jillian as our coach, we'd ALL BE SLACKING, just so we could feel her exquisite foot discipline.