Urdhva Prasarita Padasana (Upward Extended Feet Pose) registers as 1* on Iyengar’s scale of difficulty.
Iyengar doesn’t specifically counsel on this aspect of alignment within the pose, but I find it crucial to press my lumbar spine into the mat as much as possible. Only by anchoring my lumbar spine can I ensure that the pose maintains a focus on strengthening my abdomen. Iyengar mentions that Urdhva Prasarita Padasana strengthens the lumbar spine, but, I compensate for my weaker abdominal muscles by engaging my lower back, so I must deliberately drive energy into the abdominal aspect of the pose to overcome that tendency.
My tendency is a common one. Examine this pose within your own practice. Take note of your alignment as the pose becomes more difficult – especially at the point where your feet are only a few inches from the ground. The most challenging aspects of a pose call upon our full strength. Are you using the strength of your body at its greatest capacity or are you allowing the strongest muscles to take over? A few rounds of moving your legs up and down through this pose will reveal much about the strength of both your abs and the muscles around your lumbar spine. Listen to what the pose has to tell you.