Agnisara Kriya is not strictly one of the 6 main Hatha Yoga Kriyas but is the preliminary practice for Nauli - churning of the stomach muscles. It's good to practice Agnisara Kriya for a few weeks to strengthen the abdominal muscles for the practice of Nauli.

"There is a rule in Yoga that each muscle should move at least once a day. This brings our energy back into flow and releases blockages. Energy is like water. Water that stands still becomes impure and putrid. On the other hand, flowing water always remains pure. This is the reason we should always move the muscles of our abdomen and intestines daily". - Swamiji

Agnisara Kriya is not strictly one of the 6 main Hatha Yoga Kriyas but is the preliminary practice for Nauli - churning of the stomach muscles. It's good to practice Agnisara Kriya for a few weeks to strengthen the abdominal muscles for the practice of Nauli.

Agni means fire and Sara means wash so it literally means to wash the fire chakra (Manipur Chakra) located at the navel centre. The exercise also stimulates the immune system, increases the power of digestion and increases the heat in the body, burning off toxins. It is also helpful for diabetes.

“Hatha Yoga is the path of practice and purification - internal, as well as external. It is therefore an additional path to the four traditional paths (Bhakti, Gyan, Karma and Raj Yoga). Regular practice of these exercises requires willpower and discipline and that is why this path is called Hatha Yoga.
HA-THA, also means sun and moon. This refers to the union and harmony of the sun and moon principles, which result from the practice of Hatha Yoga.

There are six purification techniques that pertain to Hatha Yoga, called Shat Karma Kriyas or Shat Karmas." 

Technique:

Stand with the legs slightly apart, inhale deeply through the nose and exhale fully through the mouth while slightly bending forward and bending the knees, place both hands on the thighs. The back is straight, the head upright, allow the abdominal muscles to relax, draw the navel inwards and upwards back to the spine, avoid the common mistake of pulling in from below the sternum, don't allow the chest to sink or the back to round, hold for a moment then begin to snap the abdomen backwards and forwards 10-15 times holding the breath. Breathe out before it becomes too uncomfortable. This is one round, complete 3 rounds. It's very common at first for the abdominal muscles to tire quickly, this is a sure sign we have weak Agni.

Practise every day and gradually increase the amount of repetitions. The practice should be relaxing and stimulating and you should be able to feel the heat in your body. After a short time you will gain strength in the abdominal area and it will become very easy to incorporate into your daily practice.

Remember to practise on an empty stomach and avoid during pregnancy, menstruation, or after abdominal operations. Consult your doctor first if you have any disease of the intestine or pancreas.