Snapshots of the hotspot as it orbits the black hole clockwise on sky in a vertical magnetic field. The orbital radius is eight gravitational radii. Total intensity is shown as false colour in the background. Polarisation direction is shown as white ticks in the foreground. Their length is proportional to the linear polarisation fraction in that pixel. The hotspot samples the magnetic field geometry in time as it moves along the orbit, so that the time-resolved polarisation encodes information about the spatial structure of the magnetic field.