surely thats down to the dogs capabilities, rather than its heart, dogs cant run forever, but i agree, with the pulling up or losing heart, a dog which dont give 100% when you know it can, is what i call a jacker
I'm not sure what you mean in your first sentence; but there are dogs that don't have the capabilities to catch a good hare, but will die trying, these dogs are not "jackers"... then there's dogs that have the capability to catch a hare, and do one day, but the next, they put in a halfhearted run, and give up, even tho' they still had a chance; these dogs, to me are "jackers".