For this version, the image is multiplied by itself on a duplicate layer with normal blend mode at 30% opacity. What happens when you multiply an image by a duplicate of itself is that anything at 100% brightness stays at 100% brightness, but everything else becomes darker than its original brightness. For example, something at 50% brightness when multiplied by itself ends up at 25% brightness (with normal blend mode and 100% layer opacity). Reducing the opacity reduces the amount by which the brightness is reduced, so 30% opacity means a moderate rather than more extreme darkening effect. The original histogram is on the left and the multiplied image histogram is on the right. You can see that the multiplying effect has made the whole image darker, with the output spectrum bunched up closer to the darker end of the spectrum.

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