It's very simple and only does two things.
Optimizes caching for HDD and SAV files
Caching large files is actually harmful to your Mac's overall performance. HDD files do not need caching because the client OS will already have a cache layer. SAV files are streamed in and out and don't need to reside in the disk cache.
Always writes HDD and SAV files all the way to disk
Your data is precious. Especially data like a HDD file that took you hours to install and configure. Shouldn't you treat it that way?
I wrote it primarily because although Parallels Desktop is a great product, I got fed up with the way it wasted my entire 2GB of RAM with the HDD file and made my machine virtually unusable. With PD Tweaker installed, you will not only notice greatly improved performance from your Mac as a whole, but your HDD files will be safer from data corruption as well. And best of all, it's entirely free and the source code is available.
It uses Unsanity's Application Enhancer 2.0 to do its thing.
More information (including some rationale and a technical explanation) is available on the PD Tweaker website. What are you waiting for? Go there now!