"Up the Mountain" excerpt form Serve Doctor presents: M.P.H. http://www.youtube.com/watch?v=WlPVdppfYGs
The basic idea is many players feel the main direction of energy direction is forward, like when a pitcher throws a fastball to a catcher.
But Pat Dougherty, the Bollettieri Camp "serve doctor", explains serving is more like throwing a ball straight up into the air.
Here's a picture of Sampras ready to launch "up the mountain" out of his incredibly aggressive trophy position:
Here is a series of photos of his serve, illustrating not just his body, but his arm and racquet are directed almost straight up.
I think the upward body launch is just pretty apparent.
But notice how low he drops his upside down racquet in pic 4.
He then pulls the butt of the racquet almost straight up in pics 5 before the elbow just starts to straighten out in pic 6 initiating the racquet "flip" from upside down to right side up, and the last second pronation movement seen in pic 7.
Keeping a relaxed wrist and throwing the butt of the racquet up at the ball can't help but flip the racquet, and if you got the racquet well out to the right side of the body, resulting in pronation.
The signature pose in pic 10 with the elbow high and the racquet straight down is from not resisting that flip and pronation until the movement is complete.
THAT is "hitting up the mountain".
Of course in a second "topspin" or "kick" serve, the ball is struck at a lower point, and the ball does get "brushed up", as can be seen below:
Notice the ball is indeed rising after it leaves the racquet, as the ball is well below the top of the "24" in pic 24, and equal to the top of the "27" in pic 27.
The stop action in this photo sequence allows us to see how extension at the elbow maintains the "hit the ball with the butt of the racquet" aim until at least pic 7.
Then like Pete above, but in more detail here, you can see how a loose arm and wrist allows the racquet to flip to right side up, and to pronate from left to right - getting that a nice "slap" on the ball in addition to the "brushing up" quality.