Now, click on the Mask icon (Item 1 in red above) at the bottom of the Layers palette.

With that simple click, a brightness mask is created (Item 2 in red above) and appears beside the Layer Thumbnail.
The light areas of this mask let the bright areas through while masking (blocking) the dark areas.

So, what you've done so far is to allow the brighter areas of a 1 stop darker image to darken the blown out areas of
the bottom layer while at the same time not darkening its shadow areas. Masking is a very powerful feature.

Click 'next' in the top right corner of this page to move to the next step.