I'm not so sure it would be that easy to do with the model moving everything but their foot on a long exposure, or with combination printing. I think it was a false foot/leg with a slightly longer trouser leg flapping around in the wind.
Do you have a spare foot and leg laying around?
What makes the photograph work is that the blur of the upper body is believable. A composite photograph is often not believable; neither are contrivances like a false body part.
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Nothing beats a great piece of glass!
I leave the digital work for the urologists and proctologists.