So if you hold down your shoulder button (L if right-handed, R if left-handed) you can turn your eraser tool into a sort-of "masked subtraction mode" when you change the mode. Basically, it'll only erase some of the pixels. Shift the frame up/down/left/right and do another pass
to erase even more. Then you can copy that back to your original frame (or copy the original over top of it) to create that shadowed image effect.
So if you hold down your shoulder button (L if right-handed, R if left-handed) you can turn your eraser tool into a sort-of "masked subtraction mode" when you change the mode. Basically, it'll only erase some of the pixels. Shift the frame up/down/left/right and do another pass