The system determines the nature of the actions, and the user doesn't have to think about which mode is currently active. That's the uniqueness of the system that provides intuitive interaction with the device and new user experience.
The keyboard itself is the sensor pad. When you're typing, nothing special happens. When you swipe over the surface of the keys, however, the pointer immediately starts moving. Move it to the point you want to click, make a light touch (without pressing an actual key!), and that's a "left-click". A two finger touch performs the role of a "right-click".