Here's how to reproduce this. It's a classic...
The flash is set for fill flash. That means that what we see in the photo is a combination of the light of the flash and the ambient light. Note that in this photo the light from the flash and the ambient light are just about even. This is not a powerful flash at all. To compensate for the weak flash, the shutter speed is slow to gather as much ambient light as possible.
Someone put a hand on the girl's shoulder when the flash went off then quickly pulled their hand back right after it went off. The girl stood still for the rest of the exposure which allowed the dim ambient light from her shoulder (where the hand was) to fill in that area. They removed their hand quickly enough so that, after the flash, there's no trace of this motion.
[Looking closer, there does appear to be a blurriness to the hand that indicates motion.]
Edited by scowl, 21 November 2012 - 05:01 PM.