sdt22:
Everyone who has said "Zero RPM" has not really read Dave's first post.
...(yes, it's running)....
And the prop would show as a normal looking prop if it hadn't been spinning.
So yes, the picture was taken while the prop was turning, whether the engine was running or the wind was blowing it around. I'd incline toward the engine running.
Bill.
PS: Think about it. The shutter speed is around 1/125 second, but the strobe time is more on the order of 1/10,000 second. So the background had almost 125 times the length of exposure that the prop got. wr.