At proc time, it checks how much crit, haste, and mastery you have (yes, this is a snapshot, and does NOT include the mastery raid buff). It finds which is the highest of those 3 (tie breaking rule: crit trumps haste trumps mastery), and gives you a buff of +[sum of lowest two stats] to your highest stat, and -[lowest stat A] and -[lowest stat B]. For example, if you have 6000 mastery, 3000 crit, 2000 haste, and the proc goes off, it gives you a buff that provides [+5000 mastery, -3000 crit, -2000 haste]. It does not continue adjusting that buff as stats change during its duration. Yes, this means that if you have a temporary buff to a stat that is not your highest (even with the temporary buff), and that temporary buff falls off during Re-Origination, you *could* end up with negative rating. In this obscure edge case, negative crit does reduce your crit chance, negative mastery does reduce whatever it normally does, and negative haste is ignored.
А так работает Rune of Re-Origination