If you look in the engine bay of a petrol-engined vehicle, you will see a distributor. Sitting under the distributor cap, you will find a rotor arm. It pushes onto the end of the distributor shaft and has a protrusion pointing sideways at the electrical contacts inside the distributor cap. As it spins in time with the engine it ensures that high tension electricity from the coil travels to the correct spark plug at the appropriate time.
This device is made chiefly of electrically insulating plastic. Most of the body is cylindrical, and the bottom has a circular hole with a key to ensure it is fitted correctly. On the top is an arm sticking out sideways in one direction and the center is joined to the end of the arm with a thick metal strip.
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As the engine rotates, the contact breaker points cause the coil to generate high voltage pulses. The pulse has to be directed towards the correct spark plug, and it is the rotor arm which performs this task.
Although it is under the distributor cap, there are no electrical connections between the rotor arm and the rest of the distributor electrics; it simply takes the current from the center of the cap, and sends it out to whichever connection on the cap it is pointing to at the time of the pulse.
The most common problem with a rotor arm is forgetting to refit it after removing it. When working on distributor parts, the rotor arm often needs to be removed; it should be replaced facing the same direction as when it was removed - although the key on the shaft will make it very difficult to fit incorrectly.
The only other problems are wear and cracking of various parts of the arm. It is easy to source, and inexpensive to replace.