If hard pups don't have something to chase they will find something to bite. They are not biting out of aggression, they are biting out of prey drive. These are two totally different drives and people new to dog training confuse this all the time. They think that there is something wrong with their new family pet because it is constantly chewing on their hands and legs, when in fact there is nothing wrong with the pup except that it needs to learn manners. Prey drive puppies are very very mouthy.
Normal pups can be mouthy and bite ankles, bite wrists, bite hands, and old dogs.
A normal pup will quickly learn to respond to their handler when told to stop doing something. A hard pup can get grabbed and receive a firm shake 5 or 6 times and continue to go right back to what they were doing as if nothing ever happened. These kinds of pups require different corrections. This means they have to be allowed to play. What the owner needs to determine is what actions are acceptable and what are not.