Ashwa means horse, in this asana a similar movement of expansion and contraction of anus of horse is expected. To achieve maximum movements a specific position is taken which is called 'Ashwini Mudra".
Fold both the legs in knees.
Fold the legs in hip and press the thighs against the stomach.
Raise the entire back & buttocks off the ground.
Support the back with your hands and upper arms with your elbows rested on the ground
Contract and expand the anus in this position.
Continue normal breathing
Stop contraction and expansion of the anus
Straighten the legs at the waist
Straighten the legs in the knees and take the Sarvangasana Pose.
The knees should be just above the head or towards the ears so that the back remains straight and vertical.
The shoulders must rest on the ground.
The chin must rest in the sternal notch.
Do not bend the back.
Do not let the knees move away from your body.
Do not move the neck or overstrain it in the chin lock position.