Quarternion mode keyframes all angles to bones in quats to avoid gimbal lock. Some animators prefer this although the resulting angles are harder to edit in the graph editor. Due to time constraints quaternion mode does not support twist bones, but these are only used on destination rigs with twist bones which are rare. Also several fixes in the angle setting to deal with flipping quaternions as well as pole bones no longer being set when the limbs used to calculate the pole bones position are too straight to figure out where to put them.