I see stills at the head and tail, with video of people crazily running around with dogs in the middle.
What "combining" are you seeing?
Well... there are obviously two clips, because there is just one dog running at a time. I guess that obsticles are somehow traced and combined, so they are at the same place in "merged" video.
0:22 on that youtube clip, you can see (watch referee and that obsticle) that there are 2 clips in one.