Not GP, but the general idea is the skill to take a piece of code and understand what it does by reading the code itself (probably in an IDE that can help navigate it meaningfully), not relying on docs or explanations or anything else. Surprisingly few people are comfortable in doing this, and yet it's very common in any large software project that lots of parts of the code are undocumented and no one remembers the details of how they were written.

Precisely. I don't assume any documentation is accurate unless I've verified it matches the behavior of the code - and I've run into some doozies in my day.

The worst was a custom-built copy protection scheme that was built by a former (?) software cracker who designed it to be difficult for someone of his skills to reverse engineer. It took me a week tracing through the code to understand what it was actually doing so I could extend it to add more options.