Flexibility is only has of the equation in the yoga practice. You need a balance between the duality of strength and opening in your body and mind. Many students see a yoga posture and only recognize the element of stretching, but even the postures that demand a high level of flexibility also ask the practitioner to develop strength.
Krounchasana, the second posture of the Ashtanga Yoga Second Series, is a perfect example of a flexibility oriented posture that demands core strength. In order to get into the posture you will need open hamstrings, inward rotation, a deep knee bend and an elongated lower back. But you will also need a strong core and strong legs that are integrated into the core of the body. Starting off with the right knee bent back in half of Virasana, lift the leg by pulling the head of the thigh bone into the hip socket and activating the pelvic floor. Draw the lower ribs in and use the body to draw the leg and torso closer towards each other. Then gently grab hold of the left foot with your both hands. Instead of just lifting the left leg higher, pull down with both hands to ground the head of the thigh bone deeper into the socket. This action will further deepen the core and lift the leg on its own. Dropping the head of the thigh bone into the socket acts as an anchor to weight the base of the leg into the pelvic. Once this anchor is established the leg lifts itself and the core of the body does most of the work.