Apple PC Compatibility Card 7-inch card - Manual (Page 76)

3 Plug the connector on the short end of the cable into the cable for the monitor that you
want to use with the PC environment.
In most cases, you leave the long connector on the loopback cable unplugged.
But if your computer has an additional video card or monitor port, you can
plug the long connector into it. This will allow you to switch the monitor
between the PC environment and the Mac OS environment.
63 Connecting Monitors and Other Equipment