You open an image in Photoshop, go to Image > Adjustments > levels, and then select the eyedropper to set the blacks.
When you hover over the image with the eyedropper and press the alt key, what does this operation do?
How do I apply that action on my image so I can export it?
That’s on Windows. I don’t know if it has the same effect on a Mac.
The effect looks like posterization – it’s an ultra high contrast effect, so you can easily see the location of the shadow/highlight pixels when holding down Alt when the shadow or highlight picker tools are selected. This has nothing to do with Windows/Mac versions. Photoshop works the same on both platforms.
You could create a similar effect as follows:
Add a Posterize adjustment layer, and set the levels to 2.
Under that, add an Exposure adjustment layer.
You can then adjust the effect by changing the exposure slider.