Relative motion
This photograph was taken using a Canon EOS 90D DSLR camera with a Canon EF 35mm f/1.4L II USM lens. In order to blur the background but get the figure on the electric bike in focus I had to pan the camera horizontally, matching his speed, and I had to set the shutter speed so that it would be fast enough to capture him in focus but slow enough to blur the background. Having two moving objects in this image made the effect even better, because the viewer can see the difference in relative speed based on the bike in the foreground being in focus and the cart in the background being blurred.