So often students feel that if they can get their heels down in Adho Mukha Svanasana, they have rocked the pose. Unfortunately, that's not true. So much more has to happen before the heels get down to get the full benefit of the pose. In Light on Yoga, B.K.S. Iyengar says the "abdominal muscles are drawn towards the spine and strengthened. The diaphragm is lifted to the chest cavity and the heart rate is slowed down." In order for this to happen, the spine has to be lengthened and the legs have to be fully engaged, moving the abdominals inward and upward. (Mula Banda)
Hit your heels out against the inside of a chair and lift the chair with your heels. Feel what happens to your abdomen and observe the length of the back spine. After lifting the chair up, stretch the heels back, and then down. If you can maintain the actions you had before you descended your heels, you're good to go.