The founders of the modern Ashtanga method have been quite vocal in their conviction that asana is a prerequisite to pranayama. How many postures one can accomplish before pranayama is taught varies, depending on who you ask. If you ask me, I see no reason why simple breath exercises cannot be introduced alongside primary series. Although padmasana (i.e. full lotus) tends to be the favored among Asthangis, more than a few great yogis have explored the merits of simpler seated poses too. The legendary Goraksha Natha preferred siddhasana based on its ability to stimulate Muladhara Chakra. In essence, 11th century zealots and casual participants alike have found it to be beneficial for increasing life force energy.
Today, I decided to take siddhasana for a spin to see if it would help me get more out of my pranayama practice. Although I am not ecstatic about the benefits here, I do report a sense of calm unique to my foot position. In siddhasana one presses their heal against their perineum. The effect of this pressure helps to lower one’s heart rate and blood pressure, which could help one strengthen their breath focus. I would recommend siddhasana to anyone who feels overly frantic when performing these techniques while seated in padmasana. It also happens to be more accessible, and therefore a good starting point for those who are looking to add pranayama into their daily routine.