[en] The loss of angular momentum of a rotating plasma due to mode locking is investigated using spectroscopic rotation measurements and magnetic probe data. The electromagnetic force on the plasma regarding both the interaction with the resistive vessel wall and the error field is calculated. Simulations of the temporal evolution of the toroidal bulk rotation by solving the momentum transport equation in the presence of the magnetic forces explain the toroidal momentum balance in detail. As a result we find that the plasma within the island is affected by the electromagnetic force, while the rest of the plasma is slowed down by viscous coupling. (orig.)