How does potassium cyanide kill a man instantaneously?

In respiration, the enzyme cytochrome 'c' oxidase present in the blood binds oxygen and transfers electrons to oxygen, converting it to water and simultaneously brings about the oxidation of ferrocytochrome 'c' to ferricytochrome 'c'. When the cyanide ion of potassium cyanide enters the bloodstream, it preferentially binds to the enzyme cytochrome 'c' oxidase and thereby blocks the oxygen binding site of the enzyme.

The electron transfer necessary for conversion of oxygen to water cannot occur and the oxidation of ferrocytochrome 'c' to ferricytochrome 'c' by the enzyme is inhibited. This stops respiration, leading to instantaneous death.