Board Thread:Versus Debates/@comment-2245550-20160522071949/@comment-2245550-20160523030007

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.

So, the Sharingan doesn't copy the jutsu, you say? And Sasuke copied the Dancing Leaf Shadow with mere observation, not with training for it, which he never had the time for. And Kakashi (and Hiruko, might I add) can't copy the Rasenshuriken because it's not just controlling chakra nature, but also controlling its shape. You can learn how to perform it, but you won't be able to perform it if you haven't become capable of maintaining both the chakra nature and its shape at the same time.