A pit bulls jaws seem to lock when they bite down on something and refuse to let go.
However it's just that the pit bull refuses to open their jaws and let go of the what or who they are biting and attacking and when a pit bull attacks and bites down on something or someone it can be very hard to get them to release from it.
Usually if a pit bull bites someone the dog will have to be shot and killed to stop the attack and they are very dangerous dogs with a very strong bite.
A lot of homeowners insurance companies will refuse coverage to someone who has pit bulls for pets as they are a dog that is very prone to attacking and killing people.
A pit bulls jaws have no locking mechanism so it's not possible for the pit bulls jaws to lock but you don't want to get bitten by one because the bite can be very serious, painful and very deadly.