I shot Chris Guccione's serve this summer while he was playing doubles with Lleyton Hewitt. Chris has an enormous serve and, at 6 ft 7, can do serious damage with his serve and volley play. I took the arm action of Chris' first and second serves and compared them side by side so that we could compare the differences between the flat serve and the spin/twist serve.
The first big difference is the angle of approach to the ball. Chris is in the "Big L" position for both first and second serves, but the "Big L" for the first serve has the racket parallel with the side of the court, butt cap directly facing forward. When he prontates from this position, the racket will meet the ball flat on for a big, flat serve. Then the racket and arm will extend forward into the court as the racket and arm also continue pronating outward.
The second serve has the butt cap facing more to the left. Instead of pronating flat onto the ball and extending forward into the court, Chris will use this angled approach to twist his hand and forearm almost directly to the left - imparting both topspin and kick to the ball.