Panning is the technique of tracking a moving subject with a slow shutter speed so that the subject is relatively sharp while the background blurs into horizontal streaks. At 1/60s, a cyclist or vehicle moving across the frame renders with context — the motion of the world around the subject made visible — rather than as a freeze-frame extracted from its environment. The technique is old. What is new is doing it at 400mm.

First curtain sync fires the flash at the beginning of the exposure. Second curtain sync fires it at the end. The difference, on a stationary subject in a dark room, is invisible. The difference, on a moving subject with any ambient light at a shutter speed slower than 1/60s, is the difference between a motion blur that trails behind the subject and one that leads in front of it. The first looks like the subject is moving backward.