command merges the active layer with the layer just below it in the stack,
taking into account the various properties of the active layer, such as
its opacity and layer
The resulting merged layer will be in Normal mode, and will inherit the
opacity of the layer below. If the layer below is not opaque, or if it is
in some mode other than Normal, then this command will generally change
the appearance of the image.
The most common use of is to
construct a layer, by starting with a “base layer” (usually
opaque and in Normal mode, so that you can see what you are doing), and
adding a “modification layer” on top of it, with whatever
shape, opacity, and layer mode you need. In this case, merging down the
modification layer will combine the two layers into one, without
changing the way the image looks.
7.6.1. Activating the Command