Holy crap, do they make it so you can hack stuff with your phone in real life?
No. Its just a game in which you pretend that you are hacking the city infrastructure to help out a hacker who is running around on foot. Its supposed to be like a starter version of the mobile app that will allow you to aid people who are, at that actual moment, playing the console version.
I've set up this video link to "Jump Cut" to the relevant moment. You'll see a second screen appear in the lower-right corner. There is live video of the E3 event where this video was recorded but all it really shows is a guy picking up a tablet and poking the screen.
As you can see, the second screen is from a tablet where a "hacker" going by the handle WD_Clair helps the player out by shorting out the electrics in the helicoptor and then triggers the road blockers to raise in front of the police vehicles speeding to the scene.
If tyranny and oppression come to this land, it will be in the guise of fighting a foreign enemy. -Founding Father, James Madison