One of those lovely genetic diseases -- found in Labrador Retrievers, Chessies, and Curly-coated Retrievers, as well as in Boykin Spaniels, German Wirehaired Pointers and Pembroke Welsh Corgis.
More than you want to know here, including a video that will curl your hair. The dog in the video has the worst case of EIC I've ever seen - my Ruby is nothing like that, it takes a lot to trigger an episode.
Reader's Digest condensed version: Affected dogs have two copies of a gene that disrupts the chemical essential to transmitting nerve impulses along the spine. 10-20 minutes of hard exercise combined with intense emotional excitement and hot weather will cause the dog to lose control of his hindquarters - just as though he was paralyzed. When Ruby went down the first time, I thought she had stepped in a hole and broken her back.
As many as half of all Labs are carriers, but carriers show no symptoms. The gene was only isolated and a test developed about two years ago. The current plan is to alter all Affected dogs, and not breed a carrier to a carrier, but given the large number of carriers there's not going to be any way to completely eliminate the gene.
Also, just like with sickle cell anemia, one copy of the gene seems to confer some advantages, at least from a field trial/hunting point of view -- high drive and retrieving ability, for example. My Katy is a carrier, and she has tremendous (but controllable) drive. Ruby's drive is extreme and to some extent uncontrollable . . . at least not by me.
Check your link.