You don’t. Many people have consciousness after being beheaded. So much so that they used to pick up recently beheaded heads and show them to the crowd and they eyes would often be moving around wildly looking of shock and surprise.
According to Adler, little research was conducted on the subject, however, until the turn of the twentieth century, when another French doctor, Beaurieux, was permitted to make an investigation of a severed head, of a criminal called Languille, immediately after guillotining: “Here is what I was able to note immediately after the decapitation: the eyelids and lips of the decapitated man worked in irregularly rhythmic contractions for about 4 or 6 seconds. I waited several seconds longer. The spasmodic movements ceased. The face relaxed, the lids half-closed in the eyeballs, leaving only the white of the conjunctiva visible, exactly as in the dying whom we have occasion to see every day. It was then that I called in a strong, sharp, voice: ‘Languille!’ I then saw the eyelids slowly lift up, without any spasmodic contraction — I insist advisedly on this pecularity — but with an even movement, quite distinct and normal, such as happens in everyday life, with people awakened or torn from their thoughts. Next, Languille’s eyes very definitely fixed themselves on mine and the pupils focused themselves. I was not, then, dealing with a vague dull look, without any expression that can be observed any day in dying people to whom one speaks: I was dealing with undeniably living eyes which were looking at me.” By 1956, further research had proved, in the words of governemntal advisers Drs Piedelievre and Fournier, that “death [by decapitation] is not instantaneous every vital element survives [it is] a savage vivisection, followed by a premature burial.” The French government abolished execution by decapitation in 1977.
Awareness only ever lasted about a minute before the person was unresponsive and became unconscious, which does not seem like a long time to us. To the person who is now only a head it must seem like an eternity which is why it was a barbaric practice.