Pavanamuktasana is a great example of the "tourniquet effect" which is a tactic we exploit throughout the class. By compressing at one location in the body (in this case at the hip) we create a temporary buildup of blood which causes an increase in the blood pressure in another location (in this case, the abdominal area.) Think of it this way: your hip is the dam, and blood is building up in your abdomen like water behind the dam, pushing and pushing to get through the dam to your legs. That has the effect of stretching the blood vessels in the abdomen, improving their longterm elasticity, which is key for regulating blood pressure over time.
So as you pull your knee in to your shoulder, first right, then left the posture also aids the circulatory system in your legs, helping to flush the blood vessels and increase their elasticity by sending high-speed highly oxygenated blood down into the leg when you release your grip. That sweaty grip, by the way, is working to improve strength, circulation and mobility of the fingers, hands and wrists while toning the muscles of the arms. (The slipperier your leg is, the harder to grip- and the stronger your grip will have to become!)
Wind Removing also helps to open your hips and stretch tight lower back muscles. This posture is a classic posture used both within yoga and within physical therapy to soothe back pain. Some people even do this posture intuitively, without any instruction at all, for quick relief when their backs are hurting: It works!
One of the best ways to learn more about the postures is to read Bikram's "Beginning Yoga Class" book. Attend class 3 to 5 times weekly to put your learning into action and make your body smile!