When you enter a broken process with the debugger, you get a copy of its operand and dictionary stacks. You can examine and manipulate the objects on the stack, look at the definitions of functions and variables, and execute instructions in the scope of the broken process. You can change the stack, copy it back, and continue the process, or just kill it. You can push onto the spike the broken process, its dictionary stack, and its execution stack, and open them up to examine the process's state.
I use the debugger extensively in developing the PSIBER Space Deck, both from a terminal emulator and from the deck itself. Using the deck to debug itself is an interesting experience. One of the most common uses is to redefine a function by selecting some text in from Emacs and executing it. But it's still easy to make a mistake and crash it.
Next section, 5. The User Interface.
Previous section, 3. Interacting with the Interpreter.