Effect: the magicians borrows a watch from a spectator and whenever he said stops, the watch has stop and the secong hand has stop moving when ever he said go the second hand goes back to normal

Preparation: palm a strong magnet(I use a ring made out of strong magnet.)

Presentation: borrow a watch from a spectator. Tell them that their watch is a special watch and can make time pause(or whatever.) Quickly lay the magnet on the hand with the magnet palmed without letting the spectator seeing. But leave some gap. When you say stop make the watch closer to the magnet and the magnet should magnetted the cogs of the watch(it does no harm to the watch.) When you said go leave some gap again.