Brain debugger.
I honed mine early enough that the most I need from a program is a few Sprintf statements just to give the brain debugger some context when the application is dealing with external data or data transformation.
I personally find it hard to turn off the brain debugger and rely on software. Originally this was because you'd very much be used to the debugger being wrong. As in, "Ah, but in this case the debugger thinks X but Y is the case", and later just because the brain debugger had become so effective that not using it seemed like a lot of hard work for little extra gain.
How other programmers visualise or handle code in their brain, and how they put together new functionality in their head, is deeply fascinating. Even if you have self-awareness enough to grasp the basics of how you do it, you can be reasonably sure that each of us does it slightly differently.