Light Mummies Play Chess

There's not much chess in this picture, but I couldn't shake the question: How did the photographer do it?

That's the Spirit © Flickr user Levi Turman under Creative Commons.

The description said,

Light drawing at an outdoor chess table. Unfortunately there is a very bright light on the nearby trail, I'll want to bring a ND filter next time so I can paint slower.

Light drawing? ND filter? That raises more questions than it answers. The photographer's 'Groups' page points to Strobist.com | Flickr, which might offer further clues for the clueless, like me. For more unusual chess partners, see Big Dummies (February 2010) and Sand Dudes (June 2015), playing chess, of course.