I see, I guess I'm using "understanding" in the more literal sense of being able to put enough context together in your mind to type the characters on screen, not necessarily understand enough to know how it affects every other part of the codebase.
You don't know what these things do and what their effects really are (examples and syntax illustrative, but this is the kind of code that has disastrous effects when used carelessly), but you know they achieve your particular micro goal of "make things go fast" or "make this fit in packets on these strange industrial networks customer X has" or whatever.