Slipping Hocks are a problem that has just recently been noticed in this breed, and others as well. There is very little information on it. Infact, in veterinary orthopedic textbooks and journals, it's not even noted until after they break down, when it is referred to as "hyperextended hocks." The ligaments in the hocks are loose, making them seem "double jointed." The hocks can be bent in the opposite direction than they should be able to, and the dog has very little support in the hock when trying to jump. I uploaded a video that might make it a bit more clear...
http://www.rosecitydogs.com/slippedhock.wmv
It seems to vary from minor to extreme. I've seen dogs with the hyperextended hocks. Then there are dogs like Grant who appear to move normally, but are resistant to jumping. A friend has a dog with hocks that slip, perhaps not as bad as Grant's, yet she is a jumping fiend.