Pit Bulls were breed to control unruly cattle and large game such as boars. If the dog were to let go of the bull they could be killed and so could their owner. They same goes for a wild boar, if the dog were to let go as the hunter was approaching the boar would inflict serious and even fatal wounds upon the dog and the hunter. Pit Bulls have the will and determination to not let go no matter the circumstance, they do not lock upon their target. This determination and powerful jaw muscles is where the locking myth gets it's start. Pit Bulls and dogs that are trained for schutzhund (guarding/bite work) must let go on command, if the jaws were locked this would be impossible. Even this strong determination to hold on doesn't mean that they have a stronger bite than any other breed.
Bigman4joy, Pit Bulls do not have locking jaws as many people believe. They have the same jaws as any other dog. What they do have is very muscular shoulder and neck areas making them able to bite and hold on with sheer muscle power - but locking jaws is just a myth.
Bull dogs are awesome I'd I've never been bitten by one that wasn't in playful fun. I recommend a bull dog as a pet. They have their heart problems but they are very friendly and lovable. If I wasn't allergic I would own one.