For zappers ( Hulda Clark , Royal Rife) it is the specific FREQUENCY of electric current, which delivered to the body through electrodes supposedly can kill specific organisms, parasites, viruses etc. due to the resonance effect unique for each of them.
For Beck Blood Electrifier it is the AMOUNT OF CURRENT delivered into the blood, which (according to Kaali US Patent) can kill all known (and not known) bacterias, viruses, parasites, fungi etc. Frequency is NOT important (however most devices use 4Hz as recommended by Beck).