Unfortunately it's happened a lot to me. I used to do a lot of cold water swimming. One day I found my scrotum had tightened up so much from the cold that my testicles suddenly got forced back toward the stomach. Later I experimented with manually reproducing the effect and found that you can actually open the cavity entrance with one finger and then push the testicle into it with the other hand.
But check out this diagram and you'll see what I'm talking about.
See on the left side you've got those cords coming down from the entrance to the abdomen. Well I can get my testicles to go right back into that opening. That's probably what this guy is doing in the video.