Let's put the technique together and see how it looks with two different servers. Notice the same elements in both serves. First, both servers "hold the hit" by simply extending their arms upward by unfolding the elbow. This brings the racket into a perfect position for a hand/forearm twist across the ball.

Watch how both servers twist their hand up to the ball and the across the ball. This gets the racket to bite into the lower left of the ball and twist it up and over. Your should feel your hand and only your hand driving this motion. By twisting the hand up and over, it will cause the forearm to twist over as well.