The Window Manager forms part of the Pointer Environment and is a standard system extension. It allows you to multitask all kinds of programs easily, provides non-destructible windows and more.
One of the main problems with current versions of the Window Manager is that if any part of the windows owned by a given Job is buried under another Job’s windows (ie. you cannot see that part of the window on-screen because of another program), then if that buried Job tries to access the screen (with PRINT for example), the Window Manager will pause that Job until its window is no longer buried.
The Pointer Interface Extension (PIE) modifies the Pointer Environment so that a program is not halted when it tries to send screen output while its window is fully or partially buried by another job.
It does this by storing the changes to the buried window in memory and then when the buried Job is brought to the top of the pile (see PICK%), then its window is updated.
PIE_ON enables PIE, PIE_OFF disables it. These commands on their own cannot lead to any problems, you can switch PIE on and off whenever you like.