Get in a quadruped position.

Dip the lower back by tilting the hips (doing it as though you are pointing the tailbone up towards the ceiling) and lift the head up. Think of it as creating a valley in the lower back area.

Arch the back upward by tilting the hips (now, you are pointing the tailbone down towards the ceiling). Drop the head down.

Repeat for 10 to 20 repetitions.