This issue has been addressed before. It appears to be doing the same as if the box isn't checked but I believe it is only generating changed files. Shozam creates an awful lot of files and I think what it is actually doing is checking each file for changes. I'm creating a gallery now that has the potential of having hundreds of thousands of photos in it. I'm sure at some point it's going to get really frustrating. I'm hoping Shozam can handle what I'm creating. We'll see.
Razvan can give you a lot better explanation of what Shozam is actually doing.