In this videoJohn Petrucci tries to explain how a guy is doing a cover of one of his 7-string songs using a 6-string guitar, but I don't really understand what he is saying. Anybody can help?
He's describing the effect that happens when you play a perfect fourth with a distorted guitar. Distortion intensifies the intermodulation effect between the two strings to produce sum-tones and difference-tones.
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Basically playing the partial power-chord makes it sound like a full power-chord.