[1]A thought just occurred: how many of us actually remember what the exact steps to creating the Rasengan are? Mary's memories of the series weren't very fresh even 4 years ago, and while the Rasengan is very simple in concept, it's not quite as simple as "just spin around a lot of chakra in your hand and it'll sort itself out."[/1]
[2]And even though it's simple in concept, in execution it's extraordinarily difficult (it's A-ranked for a reason). It's not sometimes referred to as the ultimate expression of shape manipulation for nothing, after all; you have to really master shape manipulation to pull it off. [/2] [3] Naruto burned through way more chakra over the course of trying to learn the technique than even most jonin can set aside simply for training in the span of at least a month, and he still had to essentially cheat by having a Shadow Clone share the burden with him. [/3] [4] And for those saying Hisana can do it easier because of the Sharingan: Sasuke had his Sharingan when he took a month to learn the Chidori, a technique Kakashi invented after failing to learn the Rasengan.[/4]
The Rasengan (and probably any A-rank jutsu, really) is not "Okay, let's copy this thing and spend about a week ironing out the details and then we can have a big hitter for our jutsu arsenal" material. If we wanted to learn the Rasengan, it would be a genuine commitment.
Addressing indicated sections of the quote:
[1] I do, and no, I haven't reread the series, ever. The steps are: learning how to simultaneously project and rotate your chakra in multiple directions, accomplished by using a water-filled small balloon in one's hand and rotating the water in it with your chakra until it bursts, with the secret being that you need to rotate it in multiple directions at once; learning to concentrate significant quantities of chakra into your construct, done by using a robust rubber balloon and repeating first stage of training with it; learning to keep your chakra construct stable, done with an air-filled water balloon and repeating step two, but making sure that the balloon doesn't burst.
[2] Based on multiple genin-level people (Konohamaru, Naruto) learning the rasengan, it seems that, while a powerful technique, the training method outlined in
[1] is, in fact, quite efficient and allows one to quickly improve one's shaping manipulation abilities. Which is a worthy goal in itself.
[3] I'd like to point out that Naruto suffered from badly directed training. Jiraya was, very noticeably, doing the "wise old master" routine, and withholding critical pieces of information (natural direction of chakra rotation, the need to rotate in multiple directions) in order to... Improve Naruto's reasoning ability, I guess? Naruto also had sh*t control over all.
[4] Chidori is both elemental and spatial recomposition, which adds some difficulty. In one month Sasuke had to learn elemental recomposition. Remember, the thing that took Shadow Clone training for Naruto to learn, while he was older, and overall far more trained than Sasuke was? And in that time Sasuke also was able to work on his taijutsu and physical conditioning. I, however, do not think that Hisana will be able to instantly learn rasengan. However, rasengan training is a
training regiment that should drastically improve her spatial recomposition skills. And I value spatial recomposition skills a lot.