>- how come if I hold contacts for a small electrical clock with my fingers it does not work.
A small electric clock will normally only operate from a DC source of sufficient power. This power requirement would far exceed the output power of the zapper in most cases. Additionally, the zapper output is not pure DC but is a polsed signal.
>- means I have no electric current coming throgh my fingers
No, just insufficient to operate the clock FWIW.
>- How then I get the benefit?
This is explained on many if not all zapper sites.