"An ordinary battery is a source of positive voltage. It is the positive voltage that eliminates so many parasites at once, _not_ a specific frequency."
"You must be connected to both terminals... If you simply touch these terminals with your wet fingers, nothing much happens. That is because your resistance to the current starts going up right away, so less current passes through you.
However if you tap the positive terminal, your body's capacitors come into play. Capacitors only take part in the flow of electricity when they are charged and discharged. Tapping the terminal starts and stops the voltage so capacitors charge and discharge. This kind of resistance to current flow is much smaller.
The body simply resists direct current after a certain point. Tapping breaks this resistivity so that the current continues to flow. In a sense, it is still direct current (albeit pulsed) because there is no signal below the negative as happens in AC.