The siddha-asana is complicated to describe but is actually one of the simpler sitting postures. It requires less flexibility of the legs than the padma-asana yet it facilitates relaxation, concentration and ultimately, meditation.
Siddha-asana helps to establish an equilibrium throughout the body/mind. It will also help stretch the legs and pelvic area to the point where the padma-asana can be held effortlessly. Either posture, by creating a firm foundation with the legs locked in a crossed position and the spine held straight and motionless, awakens the attention and helps cultivate concentration. When concentration is highly focused and undistracted, meditation follows.