Where do pitbulls gets that determination of not letting go when they bite or fight other dogs?

Just like there are bad news about them not letting go when attacking, they also do that if they need to protect you or your family. Or it can be losing in a fight or getting beaten but even the winner gets hurts too.

Several years ago my friend's little niece (then 7 years old) was attacked by a German Shepherd. Their family pitbull (a female) jumped from the window, injuring herself in the process but still fought the Shepherd and beat and shook it badly, saving the girl. She's now 17 years old, the pitbull died of old age and she thanks the pitt for being alive.

I find it an interesting breed but is there really something special in their jaws? What do they have that other dogs don't have? Even when it comes to protecting you and if they get injured, they still hold on to the enemy, not letting go. Others dogs stopped when they're bleeding and hurt but not them.
Where do pitbulls gets that determination of not letting go when they bite or fight other dogs?
Where do pitbulls gets that determination of not letting go when they bite or fight other dogs?
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