On a Windows machine, your can quickly lower a window to the back via a Right-Click or Shifted-Left-Click into the windows's title-area.
If you dont like this behavior, evaluate:

     Display rightButtonIsLowerWindow:false

     Display shiftedLeftButtonIsLowerWindow:false
or add a corresponding line to your "private.rc" startup file.

Of course, this feature only works for ST/X windows - alien windows are not affected by these settings.
On Unix machines, this behavior is usually already provided or configurable by the window manager.