I think the TENS puts out a spike wave. It's the square wave form of zappers that uses the skin's capacitance and enables .1% of the current of the zapper to get thru the skin and into the body. Most don't realize that 5 millionths of an amp is enough to kill all the pathogens in the body. Zappers only put out five thousanths of an amp.
By the way, the fancy frequency zappers still kill by electrocution; not by frequency. I was flabbergasted when somebody started promoting that silly, unfounded concept--especially since Dr Rife hand demonstrated so clearly that specific species of pathogens could only be killed by extremely specific resonant frequencies and Dr Clark, who used all of Dr Rife's findings, was able to back that up completely and clearly distinguished the difference between freq. medicine and the electrocution of parasites. I like that the best Rife machines use flashing lights, not the square wave DC that Dr Clark promotes. The latter is still zapping, no matter what freq is used, though the very low freq's are obviously much better for killing parasites, especially the huge ones in the alimentary canal.