North Node & Karma
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I'm not sure about origin of this association in jotish, but in western astrology (which is predominantly tropic today) the symbolic importance of Lunar Nodes (both of them, not just the South or the North one - they are considered always together, because it's an axis, they are interrelated) for personal Karma is elaborated in Martin Schulman's "Karmic astrology".
look into the mithology of Rahu and Ketu, and think about the astronomical facts pertaining to Lunar Nodes, to better understand them, in general (i.e. to form your own understanding of it, instead of just accepting what's written in books).