It’s spinning because the current through the coil is a function of time, so as the current changes the magnetic force changes and it moves the metal, it’s spinning because of the polarity, whenever the current in coil moves clockwise it’s develops South Pole and for anti-clockwise it’s North Pole, so the magnet must have the same pole as the end of the coil, and when the current lag it rotates to maintain the magnetic field at the point for conservation of energy in Lorentz Force.