I could give you a textbook description of the benefits and the practice but instead I will tell you what Nauli is to me.
Shatkarma or cleansing exercise
It is the perfect start to the day! I combine the practice of Nauli with the breathing practice of Kapalabhati (see the video below). The practice is best done on an empty stomach or 10-15 minutes after a drink of warm water and lemon.
The action for me of the sharp exhilaration is one of both psychological as well as physical. I let go of any negativity as I also expel the air from my lungs. The vacuum this creates then allows me to massage the digestive system and internal organs using my abdominal muscles (which are also toned). First forwards and backwards and then right to left and then left to right.
To find the muscular isolation try and wiggle your hips as a belly dancer would. It might also be helpful to watch yourself in a mirror (if you don't get the movement at least it will give you a smile trying as you watch yourself!)
The feeling for me afterwards is one of awakening and readiness for the new day. It is the perfect prelude to any yogic practice or before breakfast. Of all the yoga practices, I try never to miss my morning Nauli!
Please remember it is a powerful practice, not to done if you have abdominal issues or if you are menstruating.