The debugger has three modes of operation: trace mode, debug mode, and zip mode. While trace mode is in force, every time you type a goal at the top level, the debugger starts single-stepping (creeping) immediately. In contrast, while debug mode is in force, the debugger does nothing until a call is made to a procedure or goal on which you have placed a spypoint. That is, it starts by leaping. Similarly, when in zip mode, the debugger begins by zipping.
The debugger mode (trace, debug, or zip) determines the first procedure call that will be traced after a goal is typed at top level. There is no other difference among the three modes. Whenever the debugger prompts you for input, you have a number of options, including those of creeping (single-stepping) leaping (jumping to the next spypoint), and zipping (jumping to the next spypoint without keeping debugging information).
The debugger mode can be set by using
prolog_flag/3 (see ref-lps-ove)
trace/0
debug/0
spy/1 also starts the debug mode if the debugger was off.