It could have been a simple elastic strap, from all I could tell from the account I read. If he had one end attached to something and the strap broke while he was pulling on it, I could easily see how it could have snapped backed into his eye and sent him flying backwards.
That might be it. He’s a former boxer and says he does a lot to stay in shape. If he was doing a boxing routine with the strap attached behind him, put all his weight and power into a punch, and the stressed part of the machine gave way, he could have gone flying forward and injured himself as he said he did.