Power from the serve (and most other strokes) comes from racquet acceleration. Work on loosening up your wrist along with using your trunk/misection to generate a whip-like effect in order to get more pop on your serve. Make sure you pronate your wrist the proper way at the point of contact. One tip I can offer is when you grip your racquet before serving, hold the grip with only 4 fingers, leaving your pinky off the grip. This will create a looser pronation that will generate mor racquet head speed. It is also helpful for tense situations.
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