Here's my technique for lower back (I tried yours, it totally worked!)
Start with lying down flat on the back, arms stretched outwards on the side. While keeping the left leg stretched straight, bend the right knee and bring the right foot on the left knee. Now, turn neck to the right and using your left hand, bring the right knee down towards the floor on the left side - as much as possible.
here's an image I found online:
… and you can repeat on the other side!