You might want to read the manual; it can be very enlightening.
Press once: If it isn't on page 1 of the home screen it will go to the home screen. If it is already on page 1 of the home screen it will go to the Search screen (that's the one with the keyboard). If it's on the Search screen it will go to the Home screen.
Press twice quickly: It will always open the Quick Launch ribbon (those aren't running apps, they are apps that you have used recently).
That's the only things that happen. Pressing twice slowly while on the Home screen will go to the Search screen, then back to the Home screen. It's really just a combination of Press once, but repeated. Likewise, pressing twice slowly while not on Page 1 of the Home screen will go first to the Home screen, then to the Search screen.