For a more comprehensive answer............

http://www.familycar.com/Classroom/charging.htm

 

I stand corrected ........ although not completely true the above statement has some truth

Following a post by Rob84 elsewhere, have had another look and it appears that with the introduction of CANBus (Hawkeye onwards??) some models are fitted with Alternators that have 4 wire connections not 3.

The additional connection is to the ECU

 

there are cutched alternator pulleys out there too, my brain cant figure out why or how they work but they are for economy and only spin in one direction. Vw have em. and renault maybe they are common.EDIT just rembered i think they are so the bigger alternators on modern cars dont ****** on the belt when you turn the engine off so arent for economy maybe just prevent wear?

 

We are talking Subaru here