When you do exercises that extend the spine, the vertebrae do NOT compress the discs as this picture suggests - quite the opposite.
Here is what really happens when you extend the spine:
What's so good about this?
It relieves axial compression hence nerve impingement. As the muscles become stronger over time, permanent increases in the disc spaces are enjoyed and pain is greatly diminished or even eliminated.
So when your PT or MD says "Stay away from arching your back!" bend over backward to send them here!