I copied and pasted the following from Unexplained-mysteries.com:
"Imagine an electromagnet, hooked up to some energy source. Attached to it by a frame is a piece of iron, at some distance from the magnet. The electromagnet is switched on for a fleeting instant, and then switched off again. Its magnetic field travels out in all directions as a wave, propagating through space at light speed, about 300,000 kilometers per second. When it reaches the piece of iron, the iron is pulled toward the electromagnet, and its own magnetic wave travels back to the electromagnet. But by the time the wave reaches it, the electromagnet has been switched off…so it is not attracted toward the iron. The forces within the system are unbalanced, and there is an “impossible” net momentum in one direction. The entire apparatus, including the piece of iron, is accelerated in that direction."