An extension of the Remote Play functionality. Firstly head over to the PS4 Link on the Vita's main menu to get the two set up, then after the Vita is recognised by the console it'll become the primary controller slot - replacing the initial DualShock and turning it off. You're free to control the PS4 using the Vita at this point, and you'll see the console's feed coming out the latter's touchscreen. However, if you then hit the Home button on the DualShock that was turned off, the PS4 essentially continues to treat the Vita as the primary controller, and brings the 'new' device on as a second pad. This way player one can continue to use the Vita as their main controller, whilst player two gets access to the original DualShock 4. Very cool.
