As the inventor of the underscore handle, let me explain. It seemed silly that if you wanted to see if a file was both readable and writable, that you'd ask the operating system twice for the same exact information (the result of calling stat on the filename). So, I suggested to Larry that there be some way to "cache" the info. He created the underscore handle to represent "whatever we stat'ed last, we'll just remember".

So, what it remembers is "the most recent stat", which includes almost everything you want to know about the file: owner, permissions, size, timestamps, etc.

Does that help?