Heh, first post and its an answer not a question
Voltage is the measurement of electrical potential, or "pressure." You can increase the voltage on these mods via potentiometer, which is dependent on the current voltage of the battery or batteries in serial, OR by having voltage boosters between the battery and potentiometer. I would not use voltage settings above 4v for a single battery, at full charge the voltage is ~4.2v. Depending on design it might not even work more than the current voltage of the battery, so an 8.4v setting really would mean that its just running at max volts available