Well, if I'm clear on what you're asking, it's about log files, right?

Well, truncating would be removing all inactive portions of the log. So your log is full with stuff it doesn't need anymore, and you get rid of it. But your physical log file that sits out on the drive is still the same size, it's just empty now. So you shrink it to fit the current size of the data. Get it?

As for defragging, that just reorgs the pages to put them back in order so they can be read easily.