When a D pad is pressed while in Advanced mode a pop up appears in the ST-16 screen that provides an abbreviated description of the control functions and requests verification that you desire the D pad functions to be active.
Short version; press the right D pad in the direction you want "cruise control" to go. The more times you press a pad the faster it will go. It generally requires 3 quick pad presses to obtain an observable cruise speed. To cancel cruise control, "still the right stick" or press the opposite direction D pad several times quickly. It's very simple, and fun, to use. You can press more than one pad in order to obtain an angular flightpath and speed.
If, after using and exiting cruise control, you experience a constant drift in a single direction during a hover, use the opposite direction D pad to cancel the drift.