Left and middle panel: reference coordinate systems and rotation angles used in the kinematical model. Right panel: toy model of a spherical expanding bubble. The origin of the reference coordinates is the center of the cavity. The velocities of the receding and approaching halves are reproduced in red and blue. The black dot marks the location of the bubble origin. The three reference axes in each figure are the X-axis (in red), the Y -axis (in green), and Z-axis (in blue). We plot the ring axis in the XY -plane (in brown).