The exact method depended on the camp. Different means of killing
-- sometimes only slightly different -- were used in different camps,
and even in different places in the same camp.
At
Auschwitz,
specifically at Krema I through III, the Zyklon-B was dropped through
holes in the ceiling. The holes are visible in
aerial photographs
that happened to be taken by Allied reconnaissance planes. At the
Action Reinhard
camps, exhaust from powerful engines, often engines stripped from
captured Russian tanks, was pumped into buildings.
There were indeed showerheads in several gas chambers; witnesses have
testified to this and wartime Nazi documents like inventories confirm
it. (See a
photograph
of the document, or
Pressac,
Auschwitz: Technique and Operation, 1989, pp. 231, 438.) It is
believed, however, that in every case, the showerheads were only
cosmetic, were not connected to anything, and that no poison gas was
pumped through them. They were intended to reassure the victims that
everything was normal, to help prevent panic as they crowded into the
chamber, and Nazis testified to this after the war.
Approximately three million Jews were gassed to death, over about
three years, at the six major death camps. The rest were killed by
numerous mass executions, mainly in the occupied eastern territories,
and in the numerous smaller camps and ghettoes by inhuman treatment
like starvation and slave-labor.
Regarding "steam chambers" and "electrocution
machinery" -- these were testimonies given by confused
eyewitnesses, in some cases Poles who were spying on the camps from the
outside. For example, someone seeing the killing process at the
Action Reinhard
camps might see the suffocating engine exhaust smoke billowing out of
the gas chambers, and mistake it for steam. Of the Nazis themselves,
or anyone else who saw the entire killing process from a close vantage
point, we know of no one who repeats these false stories.
Such stories had no evidence or corroborating testimony to back them
up, and so were not even entered as charges at the war-crimes trials. In
other words, those false stories are not evidence that the Nazis were
falsely charged -- rather, they are evidence that the trials were fair,
and that the system worked.