This pose is so called because it looks like an archer’s bow, the torso and legs representing the body of the bow, and the arms the string.
Step-by-Step Instructions

1. (don-your-AHS-anna) dhanu = bow
2. Lie on your belly with your hands alongside your torso, palms up. (You can lie on a folded blanket to pad the front of your torso and legs.) Exhale and bend your knees, bringing your heels as close as you can to your buttocks. Reach back with your hands and take hold of your ankles (but not the tops of the feet). Make sure your knees aren’t wider than the width of your hips, and keep your knees hip width for the duration of the pose.
3. Inhale and strongly lift your heels away from your buttocks and, at the same time, lift your thighs away from the floor. This will have the effect of pulling your upper torso and head off the floor. Burrow the tailbone down toward the floor, and keep your back muscles soft. As you continue lifting the heels and thighs higher, press your shoulder blades firmly against your back to open your heart. Draw the tops of the shoulders away from your ears. Gaze forward.
4. With the belly pressed against the floor, breathing will be difficult. Breathe more into the back of your torso, and be sure not to stop breathing.
5. Stay in this pose anywhere from 20 to 30 seconds. Release as you exhale, and lie quietly for a few breaths. You can repeat the pose once or twice more.

POSE INFORMATION
Sanskrit Name
Dhanurasana
Contraindications and Cautions

• High or low blood pressure
• Migraine
• Insomnia
• Serious lower-back or neck injury

Modifications and Props
If it isn't possible for you to hold your ankles directly, wrap a strap around the fronts of your ankles and hold the free ends of the strap, keeping your arms fully extended.
Deepen the Pose
You can increase the challenge of Dhanurasana by performing the pose with your thighs, calves, and inner feet touching.
Theraputic Applications

• Constipation
• Respiratory ailments
• Mild backache
• Fatigue
• Anxiety
• Menstrual discomfort

Preparatory Poses

• Bhujangasana
• Salabhasana
• Setu Bandha Sarvangasana
• Supta Virasana
• Urdhva Mukha Svanasana
• Virasana

Follow-up Poses

• Matsyasana
• Setu Bandha Sarvangasana
• Urdhva Dhanurasana
• Urdhva Mukha Svanasana
• Ustrasana

Beginner's Tip
Sometimes beginners find it difficult to lift their thighs away from the floor. You can give your legs a little upward boost by lying with your thighs supported on a rolled-up blanket.
Benefits

• Stretches the entire front of the body, ankles, thighs and groins, abdomen and chest, and throat, and deep hip flexors (psoas)
• Strengthens the back muscles
• Improves posture
• Stimulates the organs of the abdomen and neck

Partnering
A partner can help you work on a preparation for Dhanurasana. Perform step 1 in the description above. Have your partner kneel on the floor behind you, with his inner knees bracing your outer knees. Inhale and lift your upper torso off the floor by pulling your heels away from your buttocks, but keep your thighs on the floor. Your partner should then take hold of the backs of your ankles. Hang your torso from your partner's support, but be sure that he doesn't pull you any deeper into the pose. When you're ready for more, lift yourself up. Your partner's presence is merely to support whatever lift you create on your own.
Variations
A variation of Dhanurasana is called Parsva (parsva = side, flank) Dhanurasana. Perform Dhanurasana according to the instructions in the main description above. Then with an exhalation, dip your right shoulder toward the floor, strongly tug your left foot to the right, and roll over onto your right side. Students often have a difficult time rolling over for the first few times they make the attempt. Don't despair. You can practice rolling onto your side without holding your ankles. Just bend your knees and use your hands to help you get a feel for the rolling movement. Stay on your right side for 20 to 30 seconds, then, as you exhale, roll across your belly and over to the left. Stay here the same length of time, and finally roll back onto your belly with an exhalation. Parsva Dhanurasana gives a good massage to your abdominal organs.

Source: yogajournal.com