a bit of etsooi here... i started with a silhouette shot of three birds on the lamp post against a light sky... in PS i created a new layer, inverted it and set the blend mode to multiply... then i offset the top image by a few pixels and ended up with this... back in LR i adjusted the blacks and whites, and added some toner for coldth and mood... and voila!
Oh. Duh.. I didn't know there was a technical term for it. You're great at it. I wish I had the patience to get better at it but you have inspired me to try.@northy
(Thanks also for the how-to guide)