Compass Hold (MAG hold) when enabled will keep the copter pointing in the same compass direction. You can still YAW, changing the copter's heading, and the copter will then maintain the new heading. This is great for beginner flyers, they can concentrate on the throttle, roll and pitch without worrying about maintaining the copter's heading. The ACC mode, or auto-level, has been renamed to ANGLE mode and the behavior hasn't changed.