By virtue of their electrical properties, tubes generate a special waveform when they're saturated, which is why tube engineering has tremendous tonal advantages over solid state or DSP solutions, particularly for crunch and lead sounds. Tubes enter the saturation zone gradually or softly, which lends tube-driven tone its trademark yet totally unique character.
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Well i guess you should just use whatever you find is best. I find i use my wrist more than my whole arm but i have seen really fast players using their whole arm when they are shredding !