Marichyasana A

Joint movement of Marichyasana A (right):
From a seated Dandasana position, flexion in the right hip and the right knee brings the right leg close to the chest.  Extension of the right arm forward to the left of the right bended leg and protraction of the shoulder blade together with forward bending of the body should bring the right shoulder forward over the right knee.  Internally rotate the right shoulder and flex the right elbow to bring the right arm out and wrap over the right calf and thigh.  Internally rotate the left shoulder and retract the left shoulder blade to bring the left arm out and back behind the body.  Flex the left elbow and grab the right wrist with the left palm.  Further flex at the hip joint and forward bend as far down as possible over the left leg, keeping the spine straight.  Extend the neck to gaze at the left toe (dristi point).
Main muscles used to get into position:
Contraction of the rectus femoris is mainly used to flex the thigh up to the chest.  The triceps brachii and teres major extend the arm and together with infraspinatus and deltoid protract and rotate the shoulder blade to wrap the right arm over the right leg.  Brachialis flexes the right forearm and bring it over the right calf and behind the right thigh.  Contraction of the rectus abdominis and transverse abdominis help to compress the abdomen and bring the body to a forward bending position.  Contraction of the left triceps brachii, teres major together with infraspinatus, trapezius and rhomboids extends the left arm, internally rotate the left shoulder and retract the left shoulder blade to bring the left arm to the back of the body.  The flexor digitorum superficialis and flexor digitorum profundus flex the left fingers for grabbing the right wrist.  Finally further contract the rectus abdominis and transverse abdominis to fold the body deeper  and further forward over the left leg.
Benefits of the pose:
Marichyasana A is a deep forward bend with a good stretch to the spine.  It also stretches the hamstring, helps to open up the inner hip joints and improves flexibility.  It stretches and opens up the chest area and offers good shoulder opening for better posture. 
Forward bending also helps to massage the digestive organs at the lower abdomen area, mainly the small and large intestines.  The small and large intestines perform the functions of mechanical and chemical digestion of our food intake, as well as the absorption of nutrients into the adjacent blood and lymphatic vessels.  Digestive enzymes are secreted by the major digestive organs, including the small and large intestines to chemically breakdown the food molecules.  Undigested material are eliminated through the process of defecation through the rectum at the end of the large intestinal tract.
With the thigh of the bended leg pressing firmly against each side of the abdomen area, the stomach (left) and the liver (right) get compressed, added massage and stimulation.  The functions of the stomach include storage of food, mixing food with water and gastric juice to produce chyme, physical breakdown of the food contents into smaller particles and chemical breakdown of protein.  The liver (and the gall bladder just below it) is a very important organ of the body.  It produces bile (which aids in the process of the digestion of lipids), regulates blood glucose, synthesizes proteins and certain amino acids, converts ammonia (more toxic) to urea (less toxic), breaks down bacteria and toxic substances etc. 
Overall, Marichyasana A aids to improve the condition and health of our digestive system in addition to giving the body a good stretch.