Sit upright on a chair or firm surface with your feet hip width apart.
Keep your upper body and shoulders still throughout this exercise.
Arch your lower back to tilt your hips and pelvis forward so that you sit through your seat bones.
Hold this position.
Now flex through your lower back so that your pelvis tilts backwards and you are in a slumped position.
Aim to move from one position to the other with a smooth slow transition.