Board Thread:Theories and Speculation/@comment-27002073-20160406173133/@comment-2005101-20160413141511

The Genjutsu employed against Zabuza was used, at least in part, to disarm him; make it seem as though Kakashi was actually performing the copied technique even faster.

The other dōjutsu share the Sharingan's dōsatsugan, one of them possessing a far greater level of it, yet are not known for copying techniques like the Sharingan is. The Sharingan's copying mechanism isn't dependent merely on what their users can perceive, mimicking it to the best of their ability. There's something about the eye itself that literally records those details and allows the user to essentially "mirror" what is within their physical thresholds.