The Bull portion if the name comes from them being a pit "Bull" dog. The ancestors were bull dogs. Bred for bull baiting. The reason pits were bred to lock is to lock down on the bulls nose. That is the only way a 60-80 pound dog is going to take a bull down consistently. Those dogs were european. Who's breeders hold a higher regard for a dogs companionship ability than the breeders here in america did when they "invented the Pit bull terrier" using other breeds.
Todays pit bull is not the same dog. it is shorter and wider and has a completely different fighting style. Sometimes mine went in tail first let them bite them then "turn in is skin" to get a lock on them. Other times he just ran in and knocked them over with his hip. Those traits were not trained. He did them instinctively. I never trained or encouraged him to fight. They were genetic. Some fighting dog breeder generations ago came across a dog that did that, Then inbred till it was genetically set that way.