The Pin Light blending mode replaces colors depending on the brightness of the blend color. If the blend color is more than 50% brightness and the base color is darker than the blend color, then the base color is replaced with the blend color. If the blend color is less than 50% brightness and the base color is lighter than the blend color, then the base color is replaced with the blend color. There is no change to the image in areas where a dark color is blended with a darker base color or a light color is blended with a lighter base color.
The Pin Light blending mode is primarily used for creating special effects, like in this tutorial for creating a powder pastels effect. I have also seen this blending mode used to enhance shadows and highlights by applying it to a levels adjustment layer.
