This is a practice in meditation that brings wakeful/alert awareness to the subconscious. This is the point of keeping the eyes 3/4 closed. Over time you will find yourself completely alert/aware of everything happening while submerged into nothing, which is the dark void of the subconscious mind. With proper meditation this becomes easier to manage and there is never a lapse in the journey to the altered state of consciousness. However one can still progress to that wakeful alertness of an altered state of mind that includes lucid dreaming by doing the same thing before falling asleep.
While laying on your back keep your eyelids 3/4 closed. A focus point can be located on your ceiling but soon you will find yourself submerged in a dark void of nothing, but still alert and aware. The same things that take place while meditating will also take place. Simply said you will catch yourself lost in thoughts and images (daydreaming sort of). Try not to get lost into the thoughts but allow them to pass like floating clouds, while bringing your focus and attention on being there in that space "alert and aware". Unlike meditation you will probably have a lapse in consciousness and simply fall asleep. In all cases this begins to inseminate the subconscious mind with awareness/alertness, and you may begin to find yourself aware of the lucid dream state but unaware of how you got there. With continued practice you will find yourself there more often and eventually there will be no lapse in awareness with the journey there.
It is more difficult and taskful to reach the point of no lapse by simply applying the method while only preparing to fall asleep. With proper meditation it becomes easier to do and apply as with meditation you are not trying to fall asleep. The results, however, are the same in reaching an altered state of consciousness and awareness. It still takes time with effort to become effecient at doing it, and the outcome of progression is differentiated by the individual.
It is also best practiced during times of a sober mind. Meaning don't get loaded before going to sleep. You may have more profound thoughts, images, and a wild dream but the ability to recollect that dream effeciently will be hindered.