This is an attempt to emulate real CMOS cameras which reads sensor by scanlines
and hence different scanlines are sampled at a different moment in time, which
causes so called rolling shutter effect. This effect will, for example, make
vertical straight lines being curved when doing horizontal camera pan.
This is controlled by the Shutter Type option in the Motion Blur panel.
Additionally, since scanline sampling is not instantaneous it's possible to have
motion blur on top of rolling shutter.
This is controlled by the Rolling Shutter Time slider which controls balance
between pure rolling shutter effect and pure motion blur effect.