Sphinx Pose is a back bending posture that most people will find gentle. The position of the arms in this posture offers a solid support that allows you to have a good control of the movement. It does not require a high amount of strength to support the upper torso that way. This makes it easier to maintain the pose for several breaths and to experience the feeling in the lower spine.
Back bending tends to be difficult for beginners and even for more advanced yogis. A lot of time and practice are required in order the get more flexibility in the spine. Always ease into them gently and progress deeper as your muscles warm-up. This is a great asana to learn and practice before trying deeper and more difficult postures.
Always focus on lengthening the spine and not to let the vertebras compress on each other. To help this, keep the tailbone tucked and don't let the head fall back.