Sat Nam Ravi,
There are 2 basic Paths in Kundalini Yoga, one is the Path of Laya or Radiance, which is what most people know about, and the other is the Path of Pratyahar or Inward Withdrawal. In this case Yogi Bhajan uses these terms to describe whole Paths versus their specific meaning, for example a Laya Yoga Kriya or Pratyahar, as one of the 8 limbs of Raja Yoga (what the ancient yogis called Ashtanga Yoga).
In the Pratyahar Path is a specific practice called Sanjam or breath simran. This is to say slow repetitive breathing as you describe. There are several types that involve the breathing while watching the flow of the prana movement along and within the nadis, such as the ida and pingala. Others watch the flow over the top of the head where the 2 hemispheres of the brain join, another watches the flow along the sushumna, and others.
The one you describe is more typically done in a ratio of 1:4:2, which is to say that you use your left hand middle finger to close your right nostril and breath in through the left. As you breath in, you visualize the prana flowing up through the left nostril, up over the left side of the top of the head, as though following a fiber optic channel yogis call the ida nadi, where your steady awareness itself generates the light in the channel. Then down the left side of the back of the neck, down the left side of the spine to the base of the coccyx. Then hold the breath and close the left nostril with your thumb
Remember that the neck is straight and stretched upwards, chin in slightly, so you feel the stretching of the atlas and axis at the top of the spine. Also remember that the lower spine is stretched straight as well, which will cause the spine to straighten from below, with the shoulders coming back automatically. You will feel a pressure in the coccyx and an automatic pulling sensation, as though you were going to do the root lock, which comes from stretching the coccyx down (nor in and forward, but straight and down). In this way, you will be able to easily retain your attention at the base of the spine.
Sitting in this manner also causes the pranic center in the chest area projecting from the 8th vertebra down to radiate while you hold the breath in, while feeling a flow of subtle current between the coccyx and the Kundal (located between the coccyx and navel). This sensation is the location of what Yogi Bhajan calls the "reverse channels" which triggers the Kundalini through an electromagnetic sensation at the 4th vertebra. (Learning how to do spinal flex properly with power and ease from the Laya Path side of the practice is, in this regards, important.)
If you were breathing in to the count of 4, then you watch the base of the spine for the count of 16, and now you open the middle finger from the left nostril, while keeping the thumb over the entrance to the right nostril and slowly exhale to the count of 8 (the 1:4:2 ratio), as you watch and direct your awareness slowly up through the fiber optic like channel yogis call the pingala nadi, along the same path as you went down on the left side, only now your going up and over the top of the head and down to the point between the eyebrows on the right side.
Again at the point between the eyebrows, you close the middle finger over the right nostril, be sure to sit properly as described above and hold the breath, while counting to 16, and while keeping a steady flow of awareness directed through the point between the eyebrows from an area just behind and inward towards the center of the head, from where the flow of directed attention will seem to originate.
The whole process is very silent, space-like, empty, without any thoughts whatsoever, just following the flow of light that is created along these channels and centers by your own light of awareness. As though your body and surroundings are a dark space, and you are using the light of your awareness to see the life force flowing along these channels.
Having counted to 16 at the ajna chakra, i.e., the point between the eyebrows, you now do the same process in reverse, breathing in the right nostril and following the pingala, to the count of 4, holding at the base of the spine to the count of 16 and exhaling up the ida for the count of 8, and holding at the ajna chakra for the count of 16. This completes 1 cycle.
In the practice of Sanjam, which should be practiced in balance with the Laya Path practices, you should repeat at least 10 such cycles at a sitting at least once a day, better more.
Now, the feeling and sensation and experience of awareness in this practice is entirely different from the radiance of energy that you might say lasts along the spine even without attention from the Laya Path Practice of Radiance. For example, sitting with your left heel between your buttocks pressing on the sacral plexus (anal sphincter muscles between the anus ad sex organ, and with the right leg (calf) across the left thigh, knees naturally on the ground (a variance from sitting cross legged), you put your right hand behind your back with the arm pressed against your back and press down. Then you stretch your left arm up as high as possible so that the left side of the spine opens.
Pressing the shoulders back you now begin a powerful breath of fire for 3 to 5 minutes. Inhale, hold, pull the root lock, stretch the left arm up, right arm down, shoulders back, neck stretched and after 10 seconds exhale, squeeze the breath out, inhale forcefully and completely, then slowly exhale while slowly bringing the left are straight towards the side and down to your knee.
Then sit cross-legged with the hands on the knees. Now, you will feel a terrific flow of radiance, pranic life force, voltage along the left side of the spine, the back of the left side of the neck and up over the top of the left side of the head and down to the ajna, even if you don't pay attention, you will feel it. From there, the Radiance will diffuse throughout the whole body. Then wait. With every active part of a KY exercise is a passive aspect, where you just wait. During this waiting, the glands secrete to support and sustain the voltage along the spine and throughout the body.
As you keep practicing in a balanced manner, and especially keeping in mind that every KY exercise has an active and passive aspect, a pressure of voltage and radiance will begin to penetrate in and through every cell of the body.
But, as mentioned in the beginning, this radiance needs to be counter balanced with the practice of inward withdrawal, which is essentially this above described practice of sanjam - breath simran.
You will notice, especially, when you take the time to do at least 10 cycles as described above, that the mind will begin to become very subtle, extremely silent and still, like windless night, a placid ocean, and you will begin to internally see with your mind's eye, which is to say with consciousness itself the flow of light and sensation in and through these nadis. What you can see within will also come to light throughout the body and your surroundings, and you will experience yourself as a filed of light a field of consciousness of pervasive awareness.
This whole teaching of Yogi Bhajan is understood and appreciated much better, when you have a daily Sadhana. there's so much that he taught, that you can really only understand through a daily practice. Then with the practice comes an intuition of the purpose, meaning, benefit and knowledge about the practice that the world can only indicate.
This deepening also deepens the experience of Radiance.
When we come to that part of the TT manual, I'm always careful to explain these aspects in the TT program. So as teachers and practitioners, if you were not explained that the Path of Laya or Radiance is to be balanced with the Path of Pratyahar or Inward Withdrawal, then, please note it now and practice and experience for yourself.
And you will experience yourself as still, radiant, pervasive being, as Consciousness Itself." And you will know for yourself the experience and meaning of "I am the Light of the World." and know that the light that lights you, also is the same light that pervades and animates the universe and lights everyone born into it.
Then with this experience begins the inwards sense of consolidation and integration Yogi Bhajan speaks about and wishes for us which culminates in the experience of "hearing" (sunia) in which you say "I am the Truth" or as we say "Sat Nam."
So, please practice Sanjam, "Staying with the Breath" and become subtle. Transfigured.