What does the "Stop VSS...." setting do?

Hi. I could not find any information on the "Stop VSS when defragmenting NTSF volume" setting in the online help files. Please explain what it does/ when it should be used. I read the post about free space going down, but that does not explain the function, only leaves one to presume.

Thank you.

If not suspended, Windows will increase VSS size during a regular defrag. You'll lose some free space, however you can always regain free space by removing one or more old Restore Points using CCleaner\ Tools\ System Restore. I regard the selection as a minor option.

http://filehippo.com/download_ccleaner

this is for vista/7/8 only though

Thank you kroozer and Nergal.