The abdominal muscles are targeted with this exercise. In particular, the transverse abdominis (TrA) is contracting. Studies show that in normal movement the TrA contracts first before the arms and legs move. It's likely that the TrA is stabilizing the spine during active movement.
Research has also shown that patients with low back pain (LBP) have a delay in TrA muscle contraction compared to people without LBP. Rehab to regain normal muscle contraction helps decrease LBP.
There is a series of lumbar spinal stabilization exercises. You might be ready for the next level. Let your therapist know the exercises seem easy and see if you're ready to move on.