🌀 Conservation of Angular Momentum — the spinning skater (NO external torque)

There is no torque here — nobody pushes the skater. So L = I·ω is locked. Pull the arms IN and the moment of inertia I falls, so the spin ω must jump up to keep the SAME L. (Contrast with the Moment-of-Inertia scene, where an applied torque τ spins the rod up via α = τ/I.)