Just the finger? Amateurs. Proper technique involves not stopping until you reach your elbow. In the unlikely event the dog maintains it's grip, just grab whatever feels important in there and squeeze away like it owes you money.
Seriously though, a sharp blade slid into the mouth, edge facing the back of the dogs throat will either get it to let go from the pain, or you'll sever a lot of ligaments and tendons serving the jaw and get an immediate reprieve. Or just cut it's throat. Stab in behind windpipe, push edge-first away from the neck. If your pocket knife blade is 3" or longer, go through an eye for a brain kill, that might be the easiest way.
Personally, I'd do my best to stay calm while my dog proceeds to eat that dog.