While Sakin isn't really considered a harakah, we'll learn it now because of its importance. A Sakin is a letter which has a Jazm on it. And the only rule for a letter with Jazm is:

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1. It connects with the previous letter. (Alif + Baa + Sakin = Ab ) , (Alif + Jeem + Sakin = Aj ) , (Alif + Zaa + Sakin = Az )