Manipulation, isn't inherently a good or evil action. However, when it comes to Nakamura Sawa, it is the tool she wields proudly over Kasuga Takao.
The one thing I find interesting about the way she uses her tool, is how similar it is to the way Harriet Braiker describes how manipulators control their victims. Confusing Takao with a large contrast of emotions -- going from positive to negative feedback -- treating him with an aggressive tone; her intent is laid out quite clearly, and she is determined to get what she wants, with all the tools she has access to.
The other tool Sawa is using -- which is what initially started this journey of manipulation -- is Takao's social reputation as a way to keep him quiet, and actually listen to her. But that alone can only go so far before the effect wears off, and she no longer has the advantage. This is where the procedures start to pop in, and things begin to unravel as we dive deeper into the mind of Sawa.
Reward
It isn't often, but Sawa does throw Takao a bone every now and then; However, this is only if it is going to be used later or is an action she wants him to repeat. First actual reward I can remember is when Sawa allows Takao to date Nanako, and later on, this did aided in moving passed one of Takao's walls he keeps up.
And the other being a more recent case -- after vandalizing the classroom, because of the vulnerable state Takao was in -- Sawa was quite happy with the result and showed her appreciation, hand holding and approval of his actions.
Like many pets, you reward the behavior you want to see more of. Not to say Takao is Sawa's pet, but it seems like that's the way Sawa treats him -- or at the very least, he is an experiment Sawa is just trying to get to explode. And of course there isn't just rewards in this process; if there is a behavior that you disapprove of, or is just something you want to stop, you have to punish them.
Punish
We see Sawa be more neutral toward Takao, but the second emotional state for her is defiantly a negative one. Since Takao is unlikely to agree to do most of Sawa's suggestions, he needs to be pressured or even scared into changing his mind. This is probably where Sawa's personality shines best, in manipulating him aggressively, to do these types of "perverted" acts that she claims Takao loves, since he himself is a pervert.
Takao may deny it, but Sawa finds a way to draw out the true nature of him, even if it makes her look bad, in the eyes of both Takao and her classmates.
Detached Strings
And this is where the feeling of whitewater rafting comes in. You do -- for the most part -- know how you will get to the end, but it will be filled will a lot of rapids. Its somewhat predictable, yet unpredictable nature make it more exciting to see the outcome.
Lets just say, I thoroughly enjoy Aku no Hana and will continue to write about the anime on a some what regular basis -- until it ends of course. From its pretty accurate representations of the human psyche and the wonderful character that is Nakamura Sawa, I really do feel like this is going to end up being a cult classic, only because of the art style. It's too bad, but some people just are too stubborn to see an art style that isn't the typical anime style and end up refusing to watch it; but there is always the manga, as an alternative.