What is Chi Nei Tsang?
That's a good question, and don't feel bad if you have to ask - not many people have heard of it. And to your other question, it's pronounced "chee nay song" :-)
The simple answer is that Chi Nei Tsang is Chinese abdominal massage, but understanding what it's like and what it can do for you takes a little more explanation.
Chi Nei Tsang is known for facilitating that state of deep relaxation in which a person can experience their inner world, getting in touch with unprocessed emotion and bringing awareness to it. It is that moment of conscious recognition that is at the heart of transformational healing.

Chi Nei Tsang and Five Element Theory
Chi Nei Tsang loosely translates as "internal organs chi kung". It is based on the ancient Taoist philosophy Five Element Theory, as are most classical Chinese practices and art forms, such as Acupuncture, Chi Kung, Kung Fu, and Feng Shui.
In Five Element Theory, the internal organs are seen as the central storehouses and generators of the energies (the Five Elements) that run through the body. Massaging the abdomen can balance the body's energy by encouraging stagnant energy to move, discharging excess energy that is built up, and energizing areas that are depleted or drained of energy.

Chi Nei Tsang in More Scientific Terms
To put things in Western terms, we can say that the abdomen holds both physical and emotional tension, and that massage helps relieve tension. There are a third as many neurons in the gut as there are in the brain, and they connect and transmit the information of emotion using neurotransmitters like serotonin. Every feeling we have produces these chemicals. When we experience an emotion that is too overwhelming to accept, understand, and process easily, those chemicals get stuck in those neurons instead of being burned off and released. Our bodies hold on to difficult feelings in this way, and those stuck emotions can create physical tension. Chi Nei Tsang is often more about releasing unprocessed emotion than physical tension.

Healing the Unconscious Mind, Healing the Body
The body is our unconscious mind, the center of all those forms of irrational intelligence we possess: our instinct, our intuition, our emotion, and that authentic sense of self we feel when we are feeling "grounded" - that powerful awareness of our own presence, our Self, in the present moment. These are legitimate and powerful forms of intelligence, although we often don't recognize them as such. Chi Nei Tsang helps optimize our experience of these unconscious, irrational aspects of our selves.
Tension in the abdomen may effect how well the internal organs work, so Chi Nei Tsang may help those organs to function better, but I don't diagnose physical illness or practice Chinese medicine at this point.

How to Prepare for a Chi Nei Tsang Treatment:
Wear loose comfortable clothing, preferably something like elastic waist pants. You will lie on a massage table, fully dressed. You will be made comfortable with pillows and a blanket if you need one.
 - some tips: It is best to come with neither a completely full nor empty stomach. Try to avoid caffeine or other substances that might keep you stuck in tense, rational thinking and keep you from deep relaxation. It's also a good idea to set your schedule up so that you have some quiet time after your treatment to sit with whatever may come up for you. You may have some processing to do afterward (as in feeling your way through an issue to the point of understanding), and it's best not to try to do that on top of a distracting, busy schedule.

What Happens During a Chi Nei Tsang Treatment?
- health history and intake
We will start with a conversation about your health history and the specific issues that bring you to Chi Nei Tsang. I can't and don't diagnose illness, but I may ask you about things like your digestive health, sleep issues, and moods in order to get a sense of the general direction of the session. I will be looking at those sorts of complaints from the perspective of Five Element Theory, which is an energetic system, not physical.
 - the massage
Your treatment will most likely start with a short neck massage. Then we'll do some work with your breath. After that, most of the work will be massage and acupressure applied to your abdomen. I may coach you a bit in focusing your breath and your awareness. You may experience a deep sense of relaxation and re-connection with yourself on a physical and emotional level during the treatment. 
I always ask for feedback and that you let me know if anything feels uncomfortable. You always have the right to stop any massage session, for any reason.
You may fall asleep, which is fine as it is only your rational thinking that becomes unconscious during sleep - the rest of your intelligences (emotional, intuitive, instinctive, and the sense of presence that is the practical/here-and-now awareness of the Self) are still fully aware and experiencing the treatment.
The treatment will last approximately one hour.
 - good to know
Chi Nei Tsang treatments support healing of the whole person, so it is possible to bring a wide range of issues you want to deal with to the table. With this sort of holistic work, your intention has a lot of influence on your experience.
During some sessions, people get insight and guidance in the form of dreams or visions, and if that happens, it may be useful to write down whatever came to you. Pen and paper are available for you if you should want them. Be mindful of your needs and feel free to ask for anything that might support you.