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When the motherboard's clock generator pulses, the extreme values (spikes) of the pulses creates EMI (Electromagnectic Interference). The Spread Spectrum function reduces EMI generated by modulating he pulses so that the spikes are reduced to flatter curves. If you do not have any EMI problem, leave the setting at disabled for optimal system stability and performance. But if you are plagued by EMI select one of the options for EMI reduction. Remember to disable spread spectrum if you are overclocking because even a slight jitter can introduce a temporary boost in clockspeed which may just cause your overclocked processor to lock up.