Don't ask me. I've never used it, just heard about it.
Trying to read it, it just looks like a more complex version of replacing the jar, where you must put all the files manually for only one version and then take it back out. All I want is for it to switch out different versions of the .jar for me instead of going to the folder, moving files, deleting, renaming, every time I want to play with someone who's too stubborn to upgrade to pre2, which I then must repeat the process to go back to pre2 before someone else comes online and says "Nope, 1.8.1"