When you’re working on a layer mask and you want to visualize the work you’ve done, you can view the black-and-white mask by itself directly in the image. Hold down the Option (PC: Alt) key and click the mask thumbnail in the Layers panel. This even works in different layer blending modes. To show the image again, either Option-click (PC: Alt-click) on the mask thumbnail again, or simply click on the layer thumbnail for the image and not the mask.
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