A person standing on a turntable while holding a bicycle wheel is an excellent place to observe angular momentum conservation in action. The person is initially not rotating on the turntable, and the bicycle wheel is rotating about a horizontal axis.

The initial angular momentum about a vertical axis is zero.

If the person re-positions the bicycle wheel so its rotation axis is vertical, the wheel exerts a torque on the person during the re-positioning that makes the person spin in the opposite direction as the wheel. The angular momenta cancel, so that L = 0 at all times about a vertical axis.

As long as the rotating platform is well-balanced and there are no net external torques acting, re-positioning the bike wheel so its rotation axis is horizontal again should stop the person's rotation. Flipping the bike wheel over so that its rotation axis is again vertical but reversed will make the person spin in the opposite direction.