>>212596122Dogs teeth are designed to dig further into flesh as their prey tries to pull away, so when the dog bites it will most likely try thrash whatever it's biting in order to kill it, regardless of whether it will succeed. The idea is to circumvent this by forcing the dog's jaw open.
It's not perfect, it more one of those "you're situation is pretty fucked, and it's better than getting your limb shredded" circumstances.