I am regular reader of yogaforums.com and I recently found this interesting question by saumitra about Siddhasana. She experienced that while practicing Kapal bhati and anulom vilom sitting in Siddhasana position; she feel that her entire left feet numb as restricted blood flow.
Find her question in details here:
What you suggest to Saumitra, so that she can get rid from this issue.
Here is one detailed video demonstrate how to practice Siddhasana effectively.