Stellan has twisted the fella's other arm in his back, which means that he can't move if he doesn't want his shoulder dislocated on top of his finger broken. He's not really surprised to find that that is enough to get James an answer, in fact, given the type of answer it is.
It is a string of Italian words that sounds an awful lot like curses to Stellan. He doesn't know three words of Italian, so he's not one to judge, but the tone of voice says a lot in such a case. Besides, he'd be cursing too if somebody'd just broken his finger.