There are five parts of the niyamas, or "personal observances."
Sauca is for purity. Personal cleanliness. Not just outside (taking care of the body) but also there is an inward aspect. Purity of mind. Purity of the mind is even more important than physical cleansing of the body. It "is the cleansing of the mind of its disturbing emotions like hatred, passion, anger, lust, greed, delusion and pride."
I find that asana and dhyana both help me with sauca. My personal pride is on the line when doing asana as I encounter difficulty with certain postures. While I am more advance and more flexible than many of my fellow yogis (at least in the classes I take regularly), no one really cares if I fall out of a posture or do more accommodations for my body on some days. But I care. My pride is much too big. Inside my head I beat myself up when I struggle on the mat.
I struggle often with hatred, anger, lust (not sexual lust, but the lust for things or relationships I don't have), and greed (not monetary greed so much, but greed for things I don't have). In dhyana I can bring those issues to my God and lay them out. In dhyana I wait for God's acknowledgement and compassion. And in dhyana I cleanse myself through confession.