You hold the phone horizontally, like you might hold a model airplane. If you want to roll left, you roll the phone left. If you want to pitch up, you pitch the phone up. If you want to trhottle up, you swipe your thumb up on the touchscreen. Swipe right–yaw right.
The interface emulates an attitude indicator from a full-size airplane. In this picture, the pilot is commanding a right turn and an up pitch, with neutral rudder and idle throttle. In my experience, pilots look up at the models they are flying, rather than looking down at their phones. On the other hand, I enjoy eye-candy, and it’s a lot of fun explaining to other pilots how my phone translates wrist movements into flying.