It's nothing more than a registry edit. It's safe, and there's nothing to worry about. You can always reverse the changes if you so desire. It's a good way to get extra performance out of your system.
Also, remember to restart your computer after you use the program. It modifies the registry, so for the changes to take effect, you'll need to restart your system.
In the DOS days, you had to manually park your hard disk drive head with the "park" command. I used to have an XT with a 10 MB HDD, and that thing was so ridiculously expensive, that I meticulously always parked it before turning of the system. Not related but I thought it was funny to mention.