Its the same thing as OC'ing a CPU. The factory runs tests to determine how fast the chip can run, thus you may get a P42.6ghz and a P4 3.2ghz off the same wafer. In this case, the test determines how much space can be used safely while leaving extra area for bad sectors that require relocation of files.
Do NOT to this hack on your HDD. When it starts to fill up it may overwrite important data and you could completely screw up the OS and other files. All it does is confuses windows and reports the drive bigger than it really is.
WARNING 
Linear Mode
