Board Thread:Theories and Speculation/@comment-32742265-20180119154238/@comment-34223563-20180210093951

the user is able to copy almost any jutsu they see, memorising ninjutsu, genjutsu, and taijutsu with near-perfect accuracy. They can then either perform that jutsu or modify it to suit their needs, as when Sasuke Uchiha bases his Lion Combo on Rock Lee's Front Lotus. Again, Sharingan users need the prerequisite abilities before they can mimic a jutsu they have seen, and for that reason they cannot, for example, reproduce kekkei genkai abilities they do not have or nature transformations they haven't learned to perform.