Deep hip and groin opener that gets right into the joint
Stretches the back leg's hip flexors and quadriceps
Many variations to help work deeply into hip socket
Can help with sciatica
Can be uncomfortable for the kneecap or ankle. If you are stiff, the back thigh will be at a 90 degree angle to the front thigh, putting a lot of weight on the kneecap. Support the back knee with a blanket, or place a bolster under the shin, allowing the back knee to be off the floor.
Getting Into the Pose:
Begin either on hands and knees or in Down Dog. Step one foot between the hands. Walk the front foot forward until the knee is right above the heel. Slide the back knee backward as far as you can. Keep the hands on either side of the front foot.
Alternatives & Options:
If the back knee is uncomfortable, place a blanket under it, rest the shin on a bolster, or tuck the toes under and lift the leg off the floor. 
If the ankle is uncomfortable, place a blanket underneath or raise the knee by putting a bolster under the shin.
Press top of foot down firmly, emphasizing the little toe.
The first alternative pose is a simple low lunge called Baby Dragon, as shown in the picture at the top of the page. If you like, you can rest your hands on blocks.
The next option is to rest the arms or hands on the front thigh and lift the chest, increasing the weight over the hips. This is called Dragon Flying High.
A deeper option, Dragon Flying Low, is to place both hands inside the front foot and walk hands forward, lowering the hips. For more depth, come down on the elbows or rest them on a bolster or block.
In Twisted Dragon, one hand pushes the front knee to the side, while the chest rotates to the sky.
In Winged Dragon, with hands on the floor, wing out the knee a few times, rolling onto the outside edge of that foot and then stay there with the knee low. You could come down on the elbows or rest them on a block or bolster.
Overstepping Dragon exercises the ankle. From Baby Dragon, allow the front knee to come far forward and/or slide the heel backward, until the heel is just about to lift off the ground.
Dragon Splits offers the deepest stretch for hip flexors. Straighten both legs into the splits. Support the front hip with a bolster under the buttock for balance and to release weight; this relaxes the muscles. Sit up tall or fold forward for different sensations.
For Fire-Breathing Dragon, in any of the above variations, tuck the back toe under and lift the knee up, lengthening the leg. This puts more weight into the hips, increasing the stretch.
Coming Out of the Pose:
Move your paws to Down Dog position, move the back knee forward a bit, tuck the back toes under, and with a nice groan, step back to Down Dog.
A short Down Dog is delicious. Bend one knee, lifting that heel and pushing the opposite heel down, and then switch sides repeatedly.
Child's Pose feels really good after Down Dog and before switching to the other side of the Dragon.
Meridians & Organs Affected:
Stomach, Spleen, Liver, Gall Bladder, and Kidneys (and even the Urinary Bladder in the Dragon
Flying High or the Dragon Splits High).
Hips and ankles;
Lower back in the backbend options.
Recommended Hold Times:
Hold each variation for one minute and cycle through all of them.
Hold just one variation for three to five minutes.
Similar Yang Asanas:
Low lunge (Anjaneyasana). 
You may not feel anything in the outer hip joint. If your hip flexors or quadriceps are tight, that area will take all the stress. This is still a good pose, but to work your hips, other poses will be needed.
1 -- Lifting the back leg off the floor is much more advanced.
2 -- Sometimes this pose becomes the "Pedicure Fixing Asana" due to the urge to fix up the pedicure.
At these times, allow the urge to arise, but don't react to it!