You know when you are asleep, you know when you are awake. But you don't know a third point in your being when you are neither.
These two mechanisms are like two hills; you jump from one peak to another. If you remain in the middle you fall into an abyss, and the abyss is bottomless: you go on falling, you go on falling. Sufis have used this technique, and before they give this technique to a seeker, they give another practice also just to safeguard. Whenever this technique is given in Sufi systems, before it another practice is given, and that is to imagine with closed eyes that you are falling into a deep well - dark, deep and bottomless. Just imagine falling into a deep well - falling and falling and falling, eternally falling.