Turn this toggle on when you want QuickBASIC to
  - Run your program in slow motion, highlighting each
    statement as it executes
  - Record the last 20 lines of program code executed
    before a program stops
 
This lets you observe the general flow of your program.
Use with watchpoints, breakpoints, or CTRL+BREAK to
suspend program execution periodically.
 
Then use SHIFT+F8 and SHIFT+F10 to single-step backward
and forward through the last 20 lines of code.