This stretch goes deep into the groin, lengthening the entire adductor group similarly to Malasana, Baddha Konasana, and Upavishta Konasana. However, the fixed position of the legs in Mandukasana limits the external rotation of the legs which directs the bulk of the stretch into the posterior adductors (the gracilis, magnus, and brevis). Because of this, Mandukasana may prove useful for those with very externally rotated hips.
Don't know if your hips are externally rotated? Do your feet flop open in forward bends or turn out when you walk (sometimes referred to as "frog-toed")? If yes, then they probably are.
To enter Mandukasana, come down to your hands and knees. Walk the knees apart until the inner knees are in contact with the mat, then step the feet wide with the inner edges of the feet down. Begin with no less than a right angle (90 degrees) at the knee and no more than a right angle at the ankle, meaning that the feet are actively flexed. Once the legs are in position, lower down to your forearms and press the hips back to move into the stretch.
As the hips release, you can step the feet wider and/or press the hips back further to move into a deeper expression. Stay for at least one minute or as long as is comfortable.
Exit the posture slowly by first sliding the hips forward, then step the feet together and walk the knees in bit by bit.