It’s a classic device that demonstrates conservation of angular momentum and inertia. Dancers use this effect while doing pirouettes – when drawing their arms closer to the body they can spin faster.
With your arms closer to your body you can spin faster than with arms spread. This happens because of the conservation of angular momentum. You and the platform form a system. Angular momentum is a quantity related to the system’s mass, its size (those quantities make up the moment of inertia) and the rotational speed. Mass of the system remains the same, so when its size increases, rotational speed decreases and vice versa – smaller size means higher rotational speed (called angular velocity).
Depending on the model, stand or sit on the carousel. Ask someone to hand you the weights. Keep them close to your body. Ask someone to spin the carousel. Move the weights away from and closer to your body, keeping them in extreme positions for a moment. Notice how fast you spin.