from what it sounds like in the description, they basically did what i was thinking. leave the frame exposed for a few minutes to let the star trails start, have him huck, set off a flash just as he grabs. if it was dark enough, the camera would never pick up his shadow hucking before or after the flash. very cool and technical technique and definitely original. i think it's safe to say that there are many photographers out there who would love to try a shot like that, but very few who are talented enough to make it happen.
at least that's my theory and interpretation of the description.