Void pantographs work by exploiting the limitations and features of copying equipment. A scanner or photocopier will act as a low-pass filter on the original image, blurring edges slightly. It will also not be perfectly aligned with the directions of the document, causing aliasing. Features smaller than the resolution will also not be reproduced. In addition, human vision is sensitive to luminance contrast ratio. This means that if a grey region consists of a grid of very small dark dots the filtering will produce a lighter grey, while a region of larger dots will be affected differently (“big-dot-little-dot”). This makes it possible to see a pattern that previously was invisible.
https://en.wikipedia.org/wiki/Void_pantograph