It's not a piece of code I'm familiar with, and I'm making some modifications that are very major. But there's only major in certain areas - other areas work fine the way they are, and I really shouldn't touch them.
These modifications that I have to make - they're serious, and big, but mostly they can be made by just shuffling around things that already exist. A few variables, a few conditionals, a few function calls.
A whole lot of looking around to make sure I didn't just break anything.
It's like trying to do surgery on an elephant when you've never seen an elephant before, and your variable laser scalpel can be set anywhere from "vaporize individual molecule" to "bisect cubic kilometer of solid titanium".
Of course, the cool part is when you realize that you can cause a truly spectacular change in behavior by changing, oh, say, one thirty-line function.
In fact I think I can do a significant amount of the project in this single thirty-line function.
Now that's cool.