Hala means a plough, the shape of which this posture resembles, hence the name. It is a part of Sarvangasana I and a continuation thereof.
From Light on Yoga by B.K.S. Iyengar
The effect of Halasana is the same as that of Sarvangasana I. In addition, the abdominal organs are rejuvenated due to contraction. The spine receives an extra supply of blood due to the forward bend and this helps to relieve backache. Cramps in the hands are cured by interlocking and stretching the palms and fingers. People suffering from stiff shoulders and elbows, lumbago and arthritis of the back find relief in this asana. Griping pain in the stomach due to wind is also relieved and lightness is felt immediately. The pose is good for people with a tendency for high blood pressure. If they perform Halasana first and then Sarvangasana I, they will not feel the rush of blood or the sensation of fullness in the head. Halasana is a preparatory pose to Paschimottanasana. If one improves in Halasana, the resulting mobility of the back will enable one to perform Paschimottanasana well
Technique in Light on Yoga
Do Salamba Sarvangasana I with a firm chinlock.
Release the chinlock, lower the trunk slightly, moving the arms and legs over the head and resting the toes on the floor.
Tighten the knees by pulling up the hamstring muscles at the back of the thighs and raise the trunk.
Place the hands in the middle of the back and press it to keep the trunk perpendicular to the floor.
Stretch the arms on the floor in the direction opposite to that of the legs.
Hook the thumbs and stretch the arms and legs.
Interlock the fingers and turn the wrists so that the thumbs rest on the floor. Stretch the palms along with the fingers, tighten the arms at the elbows and pull them from the shoulders.
The legs and the hands are stretched in opposite directions and this stretches the spine completely.
While interlocking the fingers, it is advisable to change the interlock. Suppose that the right thumb touches the floor first, maintain the position for a minute. Then release the grip and bring the left thumb first on the floor, follow the interlock, finger by finger and stretch out the arms for the same length of time. This will lead to harmonious development and elasticity of both the shoulders, elbows, and wrists.
In the beginning interlocking will be difficult. By gradual practice of the above mentioned positions, you will interlock the fingers easily.
In the beginning it is also difficult to keep the toes firmly on the floor behind the head. If you lengthen the timing and stretch of Sarvangasana I before going into Halasana, the toes will remain longer on the floor.
Remain in the attainable pose from one to five minutes with normal breathing.
Release the hands. Raise the legs up to Sarvangasana I and gradually slide down to the floor. Lie flat on the back and relax.