cloning a selection in Photoshop with a rectangular marquee tool

Question

I've come across a new feature to me, when I Ctrl - Alt click on a selection in a layer mask, it makes a copy of that selection and allows me to duplicate the contents of that selected area and move it to another place place within the same layer mask, essentially allowing me to clone areas of a layer mask. All that happened with a rectangular marquee tool active, not the move tool, and the cursor changed to double arrows. What's "the theory" behind this behaviour? Adobe doesn't even mention that shortcut on their site.

Answer 1

Holding the Ctrl switches almost any tool to Move Tool, Alt-modifier of the Move Tool allows to duplicate current layer/selection — voilà the theory.

There are a lot of shortcuts like that: holding Alt while opening adjustment dialogs (like Levels or Curves) opens the dialog window with the last used settings; holding Alt while using Free Transform applies the transformation to a copy of a layer or selection, etc, etc. Why some of them aren't mentioned directly on Adobe site is a question for Adobe.

p.s. you don't need to hold Alt while clicking the layer mask to load it as a selection, Ctrl is enough. The same way Ctrl-clicking on a layer thumbnail loads layer as a selection (based on layer opacity).