And so you don't have to skip through it yourself: https://www.youtube.com/watch?v=VRLy0QQI6xU&feature=youtu.be...

Edit: Oh man, he sure doesn't hold back on the technical lingo.

Thanks. So, according to Carmack, the IMU Sensor in the phone is not used but they have a separate on in the frame which is faster and more precise.