In this particular case, they use a high voltage pulse that would be quite painful.
However, increasing the number of pulses will offset a lower voltage so that using 1 million pulses at a lower voltage can be as effective as a single pulse of 50,000 volts. This is much less painful and runs less risk of injury.
If you have ever grabbed a spark plug wire while the car engine was running, that will give you an idea of the power of the pulses that they are using.