LD, the rattlesnake effect is common to a whole bunch of VW PVs that use the same chips as the Vamo and SVD. They use a technique called Pulse Width Modulation (PWM) to produces the variable voltage/variable wattage effect. The only output they actually produce is their maximum voltage of six volts. If you select three volts, it actually puts out six volts, but only half of the time. If you select four volts, it puts out six volts two thirds of the time. Whether you go by voltage or power, it figures out the correct on/off ratio.
Duty cycle is about thirty Hertz or thirty cycles per second, meaning that it repeats that on/off ratio thirty times every second.
Because thirty hertz is in the audio range, lots of people can actually hear the cycle and it sounds like a rattlesnake.