One reference I found says:
Hard Mix
This blend mode is a combination of the Vivid Light mode and a posterization effect (i.e., where the image appears more pixelated). It will posterize the bottom layer pixels through the blend layer and recolor the image using the specifications of the Vivid Light mode. A higher Fill Opacity on the top layer will increase the posterization effect on the image. In the example, the Fill Opacity is set at 100%.
and Vivid Light is described so:
Vivid Light
This blend mode specifically dodges or burns the colors on the bottom layer. If the top color is darker or lighter than midgray, contrast in the bottom layer will increase or decrease.
-Rob A>