The debugger can be invoked from every screen that has a command window. Simply type /H and the debugger will be invoked.
In practice this is only useful if the debugger capabilities to set break-points is utilized. Debugging Abap coding from the PAI (Process After Input) from a screen can be a truly painfull brain breaker. Pressing enter on a screen in a transaction, could involve stepping through thousands of lines of Abap coding.
Involving the debugger can open doors, but the focus should be on finding where the bit of coding that needs debugging can be found. The debugger has the option to set breakpoints on statements. Thus if the problem is a certain message, the debugger can be instructed to set a break-point on the MESSAGE statement. After this is done, just keep pressing F8 - continue until the expected message is thrown. After that, check out debugger values, or even put a break-point in a few lines before and repeat the run.
Watch-points can also be a great help, and are often issued from a /H invoked session. More on watch-points in another article..
Note that starting the debugger can also be done from the menu: System > Utilities > Debugging Abap.