When you copy and paste something in PhotoShop you copy the pixels on a given layer and then past those pixels on another layer. Holding the Shift key while copying will copy all visible layers within a selection.
Instead of the usual “make selection” > ⌘C(copy) > ⌘V(paste), holding down shift during the “copy’ will take a snapshot of all the visible layers within the selection. Awesome right‽ Whatever is showing will be copied to the clipboard whether it’s on a background or transparent.
This can be really handy for exporting different states of a webpage, is great for cropping a transparent PNG with a shadow or simply for creating a flat version of something which can be handy for printing when you have a massive PSD.
Again, Awesome right‽