I've seen alot of Threads about this recently so i thought of this.
Maybe he unlocked his Mangekyou sharingan when he killed rin.(Alot have said this)
The reason we haven't seen it till part two.
Well we know that Kakashi has held great grief for not being able to keep his promise to obitio and his MAngekyo was a symbol of him not protecting rin so he couldn't bear to use it in part one. Till at some point (that will be explained later) he came to the realization that he couldn't let the past keep him from use this great power and could protect his current bonds using his tainted power.
Just a thought