"You are there, but you are not there." This is how my my teacher, Dr. Yang, describes the semi-sleeping state. When sitting, we are creating the conditions to drop into this state. It cannot be force, and it is a skill to be cultivated. It's like flying a kite and allowing the string to stretch as far as it can while still keeping control. This is similar to riding that line of the waking and the sleep state.
For thousands of years, Chinese have known that alongside our visible world (Yang) there exists an invisible world (Yin). This world in not viewed in the sense that we know, nor with the senses that we use on a daily basis. This world can only be felt. It's kind of like seeing the red and yellow colors of a flame, yet feeling the warmth on the skin. So often we use the word "feeling" when build this skill of "viewing" the Yin dimension.
Learning and understanding how to be "there, but not there" will lead us to deep feeling. The deeper we are able to sense and feel, the better meditators we can be. We will also be more intuitive; but for me what's most important is the realization of what is real. So I ask you, "what is real, the Yin or the Yang dimension?"