There are a few answers to this. When a joint is moved in or out of its regular positioning, gas escapes from synovial fluid in the joint. Ligaments stretching and releasing can also create sound.
According to international yoga teacher, Judith Lasater, sound comes from tendons moving across joints or from arthritic changes in the joints.
While it is usually normal, do pay attention. If symptoms change or if pain/swelling accompanies the popping or cracking noise, seek the advice of a qualified health professional. Otherwise, consider that crack a hello from your body to you!
Sphinx pose as a yoga asana or posture is a gentle back bend. Practicing this pose helps strengthen the spine and stimulate the abdominal organs. It expands and stretches the chest, shoulders, and lungs. Sphinx helps improve blood circulation while relieving the body of stress. Gentle twists and quad stretches can be added and this posture can be taken deeper into other yoga poses as well like cobra pose and raised cobra. Sphinx helps firm up the buttocks too!