We have had multiple discussions on training the spine. Here is a video clip from the clinic on how to constrain and coach a client to segmentally flex and extend the lumbar spine.
All regions of the spine should have the freedom of motion to allow for segmental flexion and extension as this is a fundamental rule of motion for the lumbar, thoracic and cervical spine.
Effective positioning of the client, creating a position that allows the client to produce progressive representations of the movement behaviour sought and effective cueing, creating a focus within the system to generate those progressive representations are very important coaching segmental movement.
Video: Lumbar Spine Training