The bully breeds are known to chomp down and hold on. They can let go when they want to. If they couldn't every such dog you ever saw in a photo of it happily swinging by it's jaws from a tire swing would need the fire department to cut it loose. Other breed individuals can do it too. My Lab does it when we play tug.
Till I tell him, GIVE.
I wonder if there is some kind of record time for tire swinging?