Note: this is my personal theory, and so it doesn’t necessarily have to conform to reality.
So why “Sleep”? Why not some other word, like “now” or even “banana”?
Let’s shortly trace through the process of an instant induction. The hypnotee focuses their attention, the hypnotist shocks them, and at the same time gives a suggestion (thus bypassing the critical factor for a fraction of a second, which allows for the suggestion to be accepted).
If we assume that this suggestion is “sleep”, then what is the subconscious attempting to do? Naturally, it tries to sleep. However the hypnotee’s conscious mind is still functioning, and thus the hypnotee cannot actually fall asleep.
In the moment in which the person tries to sleep and at the same time cannot, due to the fact that the conscious mind attempts to sleep, it bypasses it’s own critical factor.
It is sufficient to deepen this state, and thus a hypnotic trance occurs. At least that’s my theory as to why we’re using that word.
If you have any other ideas as to the function of this word, please, share. It would be very nice if I finally knew what’s going on :)