I think that Kakashi suggested Zabuza to do a jutsu that Kakashi already knew about, so that he (Kakashi) could start the jutsu before and won't have to worry about Zabuza escaping, since Zabuza will think that his jutsu will activate first...
honestly, i don't think Kakashi copied anything in this fight, he just made it seem so to get Zabuza to think he has the advange. Yes, Kakashi is that much of a badass...
Edit: i've got time (amazing what reading I's make you do, it's so boring...) so i'll explain a bit more of my Sharingan theory.
A. predictions: the sharingan can't see into the future, but rather, it showes what's probable to happen based on previous experience and muscle movements.
example: when Sasuke saw Naruto, his sharingan recorded Naruto's moving pattern, and combined with the normal human movement pattren, he could see which muscles are moving and at which angle, and his 'saw' what's the next movement Naruto's gonna do. it also works with seals, but less, since finger\hand movements are smaller and are similar for some of the seals.
think of the Ricarddo Martinez vs Ippo fight in Hajime no ippo manga, ricardo could throw super cool jabs becuase he covered his shoulder movements, and therefore, limited ippo's ability to predict punches.
it also explains why Sasuke couldn't predict naruto's 'chackra hands', he has no idea how do those things work and to which limit they can get, so his sharingan can't make him see the next thing to happen...
and to the rock-paper-scissors thing, it's really not that much of a problem to predict it, it even has a mentionning in Hunter X Hunter (GI ova, i think, maybe GI fianl), and besides, Gai probably has large rock-paper-scissors movement, so it's easier to see what's he's about to do...
well, that's all for now, i'm going back to read I's, the main charecter is like Shinji, but twenty times lamer. it's scary.
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