There is a popular game on the AppStore (That i wont mention name of here in case I get accused of promoting it). In game you have to turn on lights by doing certain things with phone. Things like plugging in earphones. Turning wifi on and off etc etc.
One of the things you have to do is suspend the game by pressing home button and returning to your home screen and pressing which ever app is in your dock at the lower left (in my case the phone app). When you press this icon it sends a some sort of feedback to the app which then unlocks the light in the game.
How? What API is used to know when user has launched another app at a certain position on the dock?
Number #36 in link to see what I am referring to