iOverwrite simply writes a large number of random-data files into the flash memory of your iPhone, iPad or iPod Touch, which overwrites any recently deleted data. This way, if your iOS device comes into the possession of someone who can perform an un-deletion of files (like hackers or other malevolent entities), your privacy is that much safer than if you had only erased your data.
iOverwrite also estimates the write speed and read speed and reports them, thereby serving as a useful benchmark of the flash memory performance.
NOTE! If iOverwrite is unable to overwrite 100% of the free space on your iOS devices flash drive, this may indicate defective TLC flash chips in your device.