This untether is a 'backup plan' from the iPhone Dev-Team. It requires access to the iOS 4.2b3 firmware and your iOS 4.2b3 SHSH blobs saved.
Currently, the untether is Mac only but the next update will bring Windows support.
Today's beta4 will eliminate separate usbmuxd process (so many more Mac users can use it). After that, expand to Windows.
You can find our instructions on performing the untether here.