Adding to what has been given below, in addition to the cross-legged posture, there's also the aspect of fixing of the eyes on the nose (nasikagra, either end of the nose) and the chin-lock (jalandhara bandha, adjunct to continuing pranayama or kevala kumbhaka). This whole posture is what is called the siddhasana, not just the leg's position.
Ancient texts say the entire nadi system is purified by the practice of siddhasana.
On Friday, May 25, 2018 at 7:52:42 PM UTC+5:30, Sheel Aditya wrote:
Siddhasana does not involve any clasping of the penis or the testicles. It is a cross-legged posture that is a variant of the normal svastikasana. The left heel is adjusted so that it adjoins/jams up against the perineum area, while the right heel is brought into position parallelly atop the left heel. The penis and testicles are adjusted in the space between the two heels (left heel is below them, right heel is above them). If the thighs and/or calves are thick or muscular, it is not possible to attain the asana position properly.
Siddhasana is primarily a meditative posture. It also has a beneficial effect on the reproductive area, but I am not certain about the ojas claim. Any effects on the generative organs are secondary.