Spinal flex (also called Camel Ride) is one of the quintessential Kundalini Yoga exercises. It is most often done while in Easy (cross-legged, seated) Pose, or Rock Pose (sitting on the heels). Inhale as you rock the pelvis forward, and push the chest forward and up. Then exhale as you round the lower back, rocking your pelvis back. This motion is repeated continuously for a few minutes. The duration will vary according to the yoga set, and to its teacher.
Here’s a video that provides a very thorough tutorial:
(courtesy of Anne Novak)