Those poses show spinal extension, not flexion. It may be the GP was using the wrong word here. I've never seen a yoga class that focused on spinal flexion, but there are a lot of poses that extend the spine, and if done wrong they can cause injury. Most yoga teachers know this, though, and encourage students not to over-extend, either by keeping abdominals active as you describe, or by focusing the extension on the mid- and upper-back rather than the lumbar.

On an only slightly related note, why doesn't gecko spell checker recognize flexion?