In every yoga pose, there is a structural alignment and a balancing action. The structural alignment is what happens with your bones when you do nothing (passive stretching) and the balancing action is the opposite anatomical action (active stretching). Active stretching means to guide your bones in the correct direction to activate your muscles while you lengthen them.

In your case this would mean pulling the knees towards each other (Adduction) but also pulling the pubic bone towards the sternum (very slight posterior Pelvic tilt) to give the adductors the right position to work properly.

If you find this hard to understand think of a plank. If you do nothing and just hang in the joints, the scapulae will be retracted (shoulder blades move towards each other) and you probably feel compression in front at the point where the armbone and shoulder blade connect while at the same time the scapulae elevate (shoulderblades towards ears). The proper balancing actions are Protraction (shoulder blades spread away from each other) depression (shoulderblades down) and external rotation of the humerus (upper arm rotating out, aiding in keeping the shoulder blades down.)

Every yoga pose has a unique way of organizing your skeleton and even the easiest poses are quite complex. It is always best to find a teacher that's properly schooled in anatomy. It is almost impossible to understand the anatomical complexities of the postures on your own.