when both devices are turned on, they each determine which is going to be the host, or Access point. once that is figured out, the other device can connect to it and use its network.
I posted some links up top, but here is another one. This tech is new so one does need to spend a little time smoking over what this does and how one can benefit.
How does Wi-Fi Direct work?
Wi-Fi Direct device connections can happen anywhere, anytime - even when you don't have access to a Wi-Fi network. Your Wi-Fi Direct device will signal to other devices in the area that it can make a connection. You can view available devices and ask them to connect, or you might receive an invitation to connect to another Wi-Fi Direct device. When two or more Wi-Fi Direct-certified devices connect directly, they have formed a Wi-Fi Direct Group, using Wi-Fi Protected Setup™ and the latest Wi-Fi security. Now you can get started doing all the exciting things that Wi-Fi Direct enables!