A 1-phase machine whose connections are flopped while running -- with the start windings no longer connected -- will treat the wave change as if the induction rotor 'slipped' and then simply keep on spinning in the same direction. This is an inertial effect.
To get a 1-phase to reverse the start windings need to be re-engaged -- hopefully after the speed drops to zero.
3-phase machines will violently reverse torque when plugged. It's an ugly thing.
