How does Memory Sweeper Pro app from the mac app store work?

It doesn't sweep files, it sweeps pages. It doesn't know which to sweep, but the operating system does. When under memory pressure, it drops pages that haven't been used in a while. This creates a large amount of free memory, but is actually horrible for performance. (Free memory is doing you no good at all. In order for memory to help you in any way, it has to be used. Every byte of memory that's free is one byte that didn't make your system any faster.)