There are a few threads that discuss this in length on the forum but the basics are:
Disconnecting the VSS wire at the back of the audio head will allow you to enter NAV guidance etc while moving. However, your NAV will not appear as smooth if you leave it disconnected as it will have to wait for updates via the satelite versus updating via the speed sensor wire.
Bottom line is, add a switch in line with the VSS at the back of the audio head and then "break" the VSS signal while you enter info, then "make" the connection again by switching back to normal mode.
read here for more info:
Plug and Play Bypass Kit (DVD, NAV and backup Camera) Discussion