Newton’s Third Law.
If you throw a hammer in space, you will fly in the opposite direction. This is how most aircraft are propelled – by throwing air behind them.
If you rotate a (heavy enough) object in space, likewise, you will rotate in the opposite direction. Consider a turntable with a pole in the middle – spinning the pole while standing on the turntable will cause the turntable to spin. It’s the same thing here.
And it’s not around the ship’s centre. It’s around the wheel’s centre – which is why you want the wheel near the centre of the ship.
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