The feature has already been explained, so I won't rehash that. But I wanted to add that when I first started using ST2, I thought it was a bug, too. I noticed that sometimes a tab showed up, sometimes it didn't. But once I got used to it, I really like it. One thing that always bugged me about TextMate was clicking around looking for the right file always opened a whole lotta tabs that I then needed to close. Now, with ST2, I can click around all I want and don't have to worry about cluttering up my tab bar. The only problem now is when I click on a file to reference it, click on some other file and then realize I still need to reference the first file again...I have to navigate to it again or Go To it. But, I still prefer the preview over automatically opening a tab.