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Very basically the aircraft's autothrottle is "programmed" to fly the aircraft at a certain speed..as DC-ATE has said in turbulence the aircraft's speed could be varying quite rapidy as the headwind/tailwind effecting it changes. The result is the autothrottle will be changing power to try and maintain the programmed speed. On most, if not all Boeings, that in turn means the throttle levers physically move fore and/or aft to reflect the power changes demanded by the autothrottle.