possibly a voltage-sensitive Relay,....volts rise, relay connects load, volts drain down , relay drops out......repeat until throttle is opened enough that the output of the alty equals the load on the system.
drive-belt must be tight-enough to transmit load.....the standard Vee-belt is designed to "wedge" into the pulley-groove. If it's too loose, it'll slip until the rubber gets warm amd sticky enough to grip and "clutch-in".
Ever started your car ona frosty morning and heard the deafening squeal,varying every time you blip the throttle, then it magically disappears?
answer as above and also, having just depleted the battery with the starter-motor, the alty is trying to shovel about 50 amps back in...the alty acts as a big brake , absorbing 1-3 horsepower to generate that current...that's why a loose belt protests.