From the factory the parts are press fit. One part must have been heated and the other frozen before being pressed together. As the parts warm and cool, they become so tightly fitted together that a hydraulic press cannot press them back apart.
With the torch technique, cuts are first made so that the ball can be knocked out of place and then relief cuts are made in the bottom of the socket to relieve stress so it can be hammered out. With practice, this can be done without damaging the bore on the arm.