When you see tyres suspended in the air, or heavy ropes hung off tree branches it's because the "trainer" is encouraging the dog to lunge and hold a sustained bite with its body weight off the ground. MOST dogs will attack a person by lunging and taking a brief bite then let go and reassess the situation, or then just run off. A dog trained to fight either other dogs or as a serious protection or military dog is trained to bite and shake, and NOT back off after one bite, but to hold and chew, whatever happens. Most dogs, even big dogs won't do this, even with training, most won't even have a proper go at a person attacking THEM, but the ones that will are the ones that do real damage to either a person or another dog.
I have taken in rescue dogs from the livestock guarding breeds all my adult life, and believe me, if you get a big dog that's trained to properly maul something, be it a predator or a person, it's a formidable opponent. These pit bull "kind of" breed types are being bred to not only circumvent the dangerous dogs act, but more concerning it's obvious there are some "breeders" actively training them to bite and hold from puppies, before selling them to anyone with the cash.
If you've got just one 120 pound fit and confident dog properly holding you that's penetrated an artery, you are in BIG trouble, if you have more than one you are lucky if you survive without skilled assistance to first get them off you, then to stop the bleeding. Ask me how I know....
But joking aside, these unscrupulous people are deliberately seeking buyers who want a potential killer, and usually the sort that buy them are into some sort of criminality or are just plain stupid inadequates, who want a fierce looking status symbol, (in the eyes of those that rate such things as a status symbol), and they have neither the environment to exercise and occupy them, or the intelligence and desire to learn what's involved in responsibly having these sorts of potential beasts.
The number of rescues I have had over the years that came from utterly unsuitable environments or owned by people with no idea what the bundle of fluff at 16 weeks will be like when it's a 2 year old "teenager" and testing its owner for who is the boss. Or has been let off the lead near livestock, (which even a livestock guarding or herding breed needs training to see as things to defend or herd, and not attack), and mauled some expensive farm animals to death.
It's often sad and sometimes disgusting to see dogs having to be shot by farmers or put down, due entirely to their owners irresponsibility, but I have also had huge pleasure from rescue work, mixed with huge frustration in seeing how some people keep their animals, and what their probable real motive was in buying (or breeding) them.
Rant over
