Pulling a knee closer while arching the back
In sitting and lying on your back, you arch your back. At the same time you are holding one of your knees (in various ways) and let it either move away from your chest or you deliberately pull it closer. This means you are rolling your legs (head of femur) around your pelvis (hip sockets), or vice versa: your pelvis around your legs.
Can you move your legs independently from your pelvis? Do you limit the range of motion of your hip joints because of anatomical constraints, stability concerns, structural damage and pain, or because of habitual movement patterns (that can be changed through learning)?