Say we have two electromagnet A and B a light year apart from each other. Someone turn on A and a magnetic field is created and propagate at c toward B. It takes a year for the mag field from A to arrive B, B is turned on just before A's field arrive. B's field made to be opposite from A and is then repelled by the A's field. But since information can not travel faster than c, when B start accelerating A would not know. It takes a year for A to notice B's field. Ordinarily, if A's magnetic field remains unchanged when B's field arrive, A would experience a repulsion force and accelerate to the other direction in comply with conservation of momentum. But what if A change it's magnetic field just before B's field arrive? Then when B's field arrive A would feel an attraction force!! This makes both A and B feel a force to the same direction resulting in a serious violation to Newton's third law. This could be used to build a reactionless drive: step 1: A turn on, mag field out step 2: just before A's mag field arrive, B turn on with a opposite field direction from A so B gets repelled. And B's mag field travel to A too. step 3: just before B's mag field reach A, A reverse current direction to reverse field direction=> A is attracted to B when the field arrive. ........... get the picture? as long as A & B switch field direction exactly out of phase, all the force acting on them are of the same direction.