This apple article explains it.
But it no longer hides all open windows for me under 10.7.4. What could be causing this?
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Command-option-click does not hide all open windows. As stated in the linked Apple article, this command hides all other open items leaving the clicked item visible. It is possible to hide all other items and minimize your most recently item with
command+option+h+m. This is a combination of
command+option+h - Hide Others and
command+option+m - Minimize.