The propeller shaft is designed and built with the yoke lugs (ears) in line with each other (1). Having the yoke lugs in line with each other produces the smoothest running shaft possible, and is called phasing.

An out-of-phase propeller shaft can cause vibration (2). The propeller shaft absorbs vibrations from speeding up and slowing down each time the universal joint rotates. Vibration is the same as a person snapping a rope and watching the wave reaction flow to the end. A propeller shaft working in phase would be similar to two persons snapping a rope at the same time, and watching the waves meet and cancel each other out. In comparison this would be the same as the universal joints on a propeller shaft. A total cancellation of vibration produces a smooth flow of power in the driveline. You must reference mark the propeller shaft before removal in order to ensure phased installation alignment. Some propeller shafts have keyed splines that keep the propeller shafts in phase during assembly.