An integral part of learning the Alexander Technique is to practise this resting balancing state. In semi-supine we are encouraging the back muscles to coordinate so that deeply held tensions can start to let go.
The intervertebral disks in our spine are subject to pressure during the day as our body weight pushes down. Cartilage and fluid in the disks are squeezed out into surrounding body tissues, as a result our stature shortens.
When the pressure on the disks is taken away, the disks have the ability to reabsorb fluids so, lying down in semi-supine actively helps the spine to plump up again.