1. A computing device, comprising: a display; a touch screen for detecting on-surface user input; a sensor subsystem for detecting off-surface objects; and memory having instructions stored on it which when run on a processor, causes the processor to perform the steps of: setting the sensor subsystem to a powered-on state, from a powered-off state, when a predetermined on-surface gesture is detected via the touch screen; determining a compound gesture using a combination of the detected on-surface gesture and an off-surface gesture determined using the sensor subsystem by: detecting an off-surface object using the sensor subsystem; determining a position and a shape of said detected off-surface object; matching the determined position with a set of reference zones and the determined shape with a set of reference poses to thereby determine an off-surface gesture state; detecting changes in the determined position and shape of said detected off-surface object using the sensor subsystem; matching said changes in the determined position with transitions between said reference zones and matching said changes in determined shapes with transitions between said reference poses to thereby determine a transition between off-surface gesture states; and, instructing a user interface to provide feedback based on the compound gesture including the determined transition between off-surface gesture states.