This effect happened by accident; I was shooting someone running past, but, because I failed to take into account the lag between pressing the trigger and the camera firing, my subject had gone and I was putting the camera down when it actually went off.

The effect is created purely by camera movement and a relatively low shutter speed; I just converted to black and white and upped the contrast to enhance the efffect. It reminds me a bit of Mario Giacomelli's pictures of priests having a snowball fight.