How to Create the Navigation Window and Work with it
Hold down the Option key and choose View > Full + Monitor (the first item of the View menu).
The document window becomes small (it behaves like a navigation window) and a monitor window is created (it fills most of the screen). The monitor tool is selected and the cursor takes the shape of a little cloud.
Select a region into the upper window and it will be displayed inside the bottom window.
It's the monitor tool that performs this action. If the monitor is not the current tool, you can obtain the same effect by selecting a region and pressing “m” on the keyboard.
You can do nothing directly into the monitor. Press 5 to swap the roles between the two windows: now you can manipulate the expanded region, for example you can integrate it. Press 5 again to swap the roles back.
You can resize both windows, if you like. The command Full + Monitor is only a shortcut to arrange the two windows in a certain way. You can get the same effect with the generic command Tools > Monitor, which does not resize the main window.
To return to the normality, close the monitor (click its red button) and zoom out the navigation window (click its green button).