Left-drag to rotate q1 or q2. You can also rotate the reference frame in
the top left to get a better look at q1 q2, dq, and Dq.
"dq" (q2 / q1) is the quaternion taking q1 to q2 in the stationary reference frame;
"Dq" (q1 \ q2) is the quaternion taking q1 to q2 from the perspective of the q1 frame. Mathematically: