Parsvakonasana (Side Angle Pose) is a foundational standing posture with a long, lateral stretch from heel to fingertips. I'm demonstrating the extended variation, (Utthita Parsvakonasana or Extended Side Angle) in the picture above, but a variety of arm variations may be used to make the pose more accessible or focus the intent behind the posture.
The front thigh and outer hip work to support the weight of the body in this pose as the inner thigh and groin are stretched. The bottom hand presses into the floor to broaden the chest and spiral the torso open. The bottom side of the torso contracts strongly, strengthening the muscles of the side and back while lengthening the same set of muscles on the opposite side, improving lateral mobility.
To enter the Parsvakonasana, take a wide stance, turn the back toes in, and bend the front knee directly over the ankle. Ensure that both the toes and the knee of the front leg are pointing straight ahead and ground evenly through all four corners of the front foot. Press the entire sole of the back foot down by engaging through the inner thigh and engage the buttocks to roll the top hip open. Draw the shoulderblades onto the back to create space between the shoulders and ears, releasing any excess tension in the neck. To modify the posture, take the elbow to the top of the thigh instead of pressing the hand into the floor. To deepen the posture, firm the front knee against the upper arm. This action will assist in further opening the chest.