In a real project - millions of lines of code - there is no chance to understand it all. You have to trust the abstraction does what it says without caring how it works. You only break the abstraction and understand when you have reason to suspect that it somehow isn't obeying your quick mental model of what it does - normally this means you suspect the bug you are working on is down that abstraction, but sometimes it is just the name is bad and so you need to figure out what it is really doing.