Crowbar? (was: Re: Testing Power Supplies!)

[O. Sharp]

Tony Duell wrote of crowbar protection circuits thus:

> I'd rather not trust a protection device if I don't have to. Sure I like 
> crowbar circuits, and they have saved my chips a few times, but I don't 
> like to depend on them.

Ah, twice in one week I get to expose my wanton ignorance.  :)  What 
typically makes up a crowbar circuit? How are they triggered? If I'm 
understanding this correctly, a crowbar quickly kills the PSU and then 
drains power from the system rapidly and nondestructively... but how it 
accomplishes this, or knows it _should_, I don't yet understand.

O'course, one could accomplish this with an actual crowbar applied 
across the PSU output... but I suspect this would not be described as 
"nondestructive".  :)

                      -O.-