It is said that successively visualizing these four words will cause the Four Voidnesses to emerge successively. However, this saying makes little sense, because some do not understand that in meditating on these words at the Heart Center in the Central Channel, the life prana will automatically gather; consequently, the Four Voidnesses will arise. Based on this reasoning, one should not misinterpret the text "visualizing the word of the center at the Heart." The essential thing is to visualize at the central point of the Heart Center in the Central Channel. Because the words of the instructions are not clear, it is wrongly said that concentrating on the three words on the three leaves will produce the Three Voidnesses. This opinion completely misses the essential point of the teaching. Therefore,

we may dispense with this Ah, Nu, Da, Ra word-practice, which is difficult and results in little benefit.

The important thing is to concentrate on the Hum word in the center.

There are two different kinds of meditator of the Sleeping Light: the yogi who has attained a stable Samadhi of the visualized object before (engaging in this practice), and the yogi who has not attained a stable Samadhi. In the latter case, the yogi will attain a Sleeping Samadhi immediately after he falls into sleep through holding his mind on the instructions given before, but this Sleeping Samadhi will last only a very short while. Therefore, relying merely on the mindfulness of recognizing the sleeping state, through desire will not give one a stable Samadhi even though great efforts are made. For if one cannot "hold" the Light through the prana power, the inhalation and exhalation will still take place. Though the breathing may be very subtle and calm, the breath cannot be completely stopped. Thus a counterpart of the Fourth Emptiness will emerge but not the real Fourth Emptiness. Consequently, the Sleeping Light cannot be (fully) recognized.

In the case of the yogi who has already attained a stable Samadhi and has mastered the Heat Yoga capable of gathering the Life Prana in the Central Channel and raising the Fourth Emptiness, he will have no difficulty in unfolding the Light-of-the Sleeping-State. If he follows the instructions as given before and visualizes the Hum word in the Central Channel at bedtime, he will be able to unfold the Fourth Emptiness of the Sleeping State through the dissolving process of prana within two or three days.

If the yogi cannot unfold the Emptiness through the prana power in the daytime, but if he attains a stable Samadhi