Downward Dog vs. The ShanMonster

Hoorah! Yoga didn't kill me this morning! I took my asthma inhaler before class, and this, combined with the fact the air exchanger was on in the studio this time, seemed to make a heap of a difference. I felt rather stiff through the hamstrings today, so my downward dog asanas weren't so hot. Also, there's this other asana (whose name I forget) which gives me grief. We start sitting on our calves, then tuck our foreheads down tight between our knees before hoisting our arses in the air. How do I do this without squashing my eyeballs on my kneecaps? I think I lack the flexibility in my back to get this manouevre right.