The magician places a hand over a spectator's watch and summons a great burst of mental power. When the magician's hand is removed, the spectator looks at the watch and it has stopped running!
The magician is wearing a ring that is not really a ring at all but actually a high-capacity capacitor chock full of electricity. When the magician places a hand over the watch, the ring is triggered and some 7,000 joules of electric energy surge into the timepiece, fusing the workings of a mechanical watch or blowing out the circuitry of a digital one.
If the spectator complains about the broken watch after the effect, the magician reminds them -- it's not a broken watch -- it's a souvenir!