Use the long edge of a towel and sit it across the affected level as indicated by your therapist, with one edge of the towel/pillow case in either hand.
Apply a comfortable but firm pressure over the low back by pulling the ends of the towel/pillow case whilst keeping your thumbs pointed upward. You may add a gentle elbow bend to increase the level of pressure.
Ensure you are ulling in an upward diagonal.
Whilst maintaining this pressure, gently lean forward in your chair.
Once at the ned of your comfortable range, slowly rise back up.
Complete and modify as prescribed by your physiotherapist.
Precautions
Ensure your feet are firmly planted on the floor throughout
If you experience an increase in pain, cease the exercise and contact your physiotherapist