For whatever reason, it was far easier to do this on my phone than on my laptop, even after trying to fiddle with various combinations of zooming in/out and adjusting my distance to the screen. In some cases, it can also be easier to do without glasses (if you customarily wear them) as it makes the defocusing easier.

The effect itself is basically similar to the Magic Eye stereograms (https://en.wikipedia.org/wiki/Magic_Eye) which by themselves are pretty neat; your brain can rapidly detect subtle offsets in random patterns to reconstruct depth cues. In the case of “spot the differences”, the shimmering is due to the irreconcilable disparity between left and right images, which manifests visually as a glaringly obvious “unphysical” apparition - e.g. flickering between the left/right images, or appearing to be out-of-plane with the rest of the picture.