The Sanskrit word pavana means air or wind and mukta means freedom or release, therefore this is the "wind relieving posture" so named because it assists in releasing trapped digestive gas from the stomach and intestines.
"Standing straight on the left leg, bend the right leg and place the right foot on the root of the left thigh. Stand thus like a tree on the ground. This is called vriksha-asana." Gheranda-samhita II.36
The Wind-relieving Pose Instruction:
Lie flat on the back in the shava-asana.
Inhale and bend the right knee and pull it close to the torso with both hands while interlocking the fingers just below the knee. Keep the left leg flat on the floor.
Hold the inhaled breath for a few seconds then exhale slowly through the nostrils and lift the back, shoulders and head off the floor and touch the knee with the forehead.
Hold the exhaled breath for a few seconds then slowly inhale and return the back, shoulders and head to the floor. Remain holding the knee.
Hold the inhaled breath for a few seconds then exhale while bringing the right leg to the floor.
Lie flat on the back in the shava-asana for a few seconds then repeat beginning with the left leg.
As mentioned above, practicing the purvana-mukta-asana will help to release gastrointestinal gas. It is also improves other gastrointestinal problems like upset stomachs and constipation by stimulating the abdominal region.
Hold each of the four parts of this posture (corresponding with the held breaths) for three to five seconds each. Repeat the purvasa-mukta-asana three to five times on each side.