Board Thread:Naruto Discussions/@comment-24824148-20160724215526/@comment-732150-20160727220741

Kakashi clearly had knowledge of what MS was in Part I, because the second he understood Itachi had it, he warned Asuma and Kurenai. The fact his MS was called new when we first saw him using it against Deidara shows that he only recently learned how to use it. Considering he fainted immediately after awakening it when Rin died, he probably didn't remember he had, or possibly didn't even feel it awakening. Sasuke himself forget he awakened the Sharingan during the Uchiha clan massacre. During the timeskip, Kakashi probably did some logical thinking about MS, concluded that he met the criteria for awakening it, and tried using it.