Ferlin - it's one of those things that you use in very specific instances. I'll try to give better details... hmm...
Firstly, I was talking about turning
off the "all layers" toggle. Layer 1 is say... a photograph. Layer 2 would be something that is transparent, or has the blending mode turned down. Then you can clone effects from layer 2 onto layer 3. This allows you to duplicate portions of layer 2 if you're filling in effects and such.
It's one of those things that you have to experiment with and then tuck away until you reach a situation that requires it. Definately not as useful as the technique Erik mentioned, but thought I'd pass it on as another facet.
Didn't mean to be confusing...well... more confusing than usual. heh