The reason why some alternator lights don't go out until you rev the
engine a bit, is that the regulator needs to hit it's specific value to
start charging. I found it hard to believe when I first heard it, but it
makes sense. How it all happens, I really dont know!

The output on the alt gives grounds to the light at rest, i.e. there is
a drain. If the bulb goes out, the bulb wont use any current, and the
alt doesn't start to charge. That's why cars use a LED instead these
days. When the alt is charging, the bulb or LED gets positive both ways,
but if the alt goes bad, the light gets ground from the alt again, and
lights like a full moon.

When the regulator starts to charge, the light will not light up again
even if the alt goes under the specific charger value, because the
"barrier" in the alt has been met.

PerL (Not an electrician)
87 Syncro 112i