so i was poking around Autodesk Sketchbook and i came across a setting called 'overlay'. it seems to me a bit like multiply, but slightly different. i've also seen it around WolfPlay. anyone care to explain?

Overlay is a layer blending mode that is basically a combination of multiply and screen. It accentuates both the dark and light values by multiplying or screening them respectively. The brightness is unchanged so it tends to look harsher than something like Soft Light for example. I personally use it for things like the mood of a portrait as different hues can give different feelings to the painting.