After you’ve made a selection, tapping the Q key will toggle you between the selection and Quick Mask mode, which is a mask on which you can paint to modify your selection. But there’s another hidden advantage. When you’re in Quick Mask mode, you can use the selection tools on the Quick Mask. This means you can make selections within selections—very inception-like. Now you can fill those selections with black, white, or gray to remove, add, or partially add to the mask, respectively. Just imagine the possibilities! 

This tip previously appeared in Colin Smith’s Photoshop Tips column, in the May, 2019 issue of Photoshop User magazine.