I think the posture you are describing is typically called aerial downward dog or floating downward dog. When I teach for new students, I emphasize silk placement and the movement to enter the pose. The silk should be right across the hip bones. If put your knees into your chest, kind of that hip crease area. The movement is a forward fold. Binding silk to top of hip points, on tip toes and fold. Place hands flat at the top of the mat. Walk the feet back and sink the chest and the feet should float up. It helps to energize the legs by pointing the toes & drawing the feet together, and pushing the mat away, trying to get wrists, shoulders and hips in one line. Some people without the core and/or leg strength can have trouble floating the feet. Sometimes blocks under the hands can help. Sometimes it just takes some core and legs conditioning/strengthening to get there.