locking up the monkey heart and tying up the running horse of intellect: The alchemical mystery of the Small Universe bellows as the Microcosmic Outer Alchemical Agent

This is called inner and outer ho p’i (i.e. inner vital force 
and outer air) which operate like a bellows. Although the 
mechanism is the same the air in the body works differently 
from that in the bellows. While the inner breeze starts the 
eyes roll to look at the four cardinal points A, D, G and J, 
and while nature and life in the body change place during 
the shutting and opening process, the latter sets in motion 
the generative force which is automatically transmuted 
into vitality (which circulates up and down). Even the 
practiser himself does not know why the head, eyes, limbs 
and breathing function like this, for each inhalation followed 
by the shutting, and each exhalation preceded by the 
opening, takes place spontaneously.

 So the physical breathing turns the generative fluid produced by food digestion into generative force (yin qi) and the Spirit or Fire from the heart transforms the generative force into Yuan Qi or prenatal vitality. So the cleansing and purification "stops" and "restarts" and "rests" at the 4 directions - the kidneys, the heart, the third eye and the lower tan t'ien. If the generative fluid is not purified then it sullies the blood - and so clogs the channels.

So then the eyes rotate with the breathing so that the Sun and the Moon are mingled and turned inward - so that "positive fire" is created which then can create the Yuan Qi from the generative force. The generative force thus turns into the alchemical agent and "positive principle" (Yuan Qi) is restored for immortality.

Then it is by spirit awareness that we determine how much Yuan Qi is built up, as the microcosmic inner alchemical agent, which is stored in the "inner" core of the Lower Tan T'ien.

The yang qi, going up the spine, creates light internally as the "relative void" but the yin qi and yang qi are in the "outer" core of the lower tan t'ien. So they need to be further purified by the Positive Spirit of the Sun and Moon to keep building up Yuan Qi. Once the Yuan Qi builds up enough, from enough generative force being restored - back to a 16  year old, then the third eye fully opens and the T'ai Chi is experienced as a bright white light:  The Light of vitality, the Yuan Qi, manifesting the Yuan Shen, outside the body, as the Mysterious Gate, the Dao.

So this combination of Spirit plus Breathing being synchronized is called

locking up the monkey heart and tying 
up the running horse of intellect 

If the Small Universe or Microcosmic Orbit meditation is then not continued, then the Yuan Qi can not be stored up and the progress is lost, the "Foundation" will be lost, and the person "falls out of enlightenment."

If the intellect (horse) and passion (monkey) can not be cut off then at best only the generative force (yin and yang qi) can be maintained for health.