The "one wire" alternator is one that only uses a single wire that goes from the alternator to the battery (at the alternator I believe a jumper is used for voltage sensing), but these alternators require a little more rotational speed before they "kick in" than the normal 3 wire set-ups. Also, typically, the larger the alternator (amp wise), the higher the rpm that they kick in at (1300-1500 rpm). Once they've kicked in, they will still charge the system down below the kick in speed, and on the smaller ones may countinuously charge until the engine is turned off.