To convert your stamina into chakra(make chakra) necessary to perform any ninjutsu or genjutsu techniques
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On the above pic for example Naruto converts 30% of his stamina into chakra and thus in this case, the remaining stamina is 70% with 30% of chakra available for ninjutsu or genjutsu techniques
Ninja need to convert stamina into their chakra perfectly to avoid chakra waste in the end. Someone like Sakura for example can
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But Sasuke while being inept in chakra building himself can't avoid a chakra waste. It's for that reason that during the process of converting his stamina into chakra, if 30% is what he needs in reality, he would be converting 40% of his chakra instead, and thus leaving a 10% of chakra as extra
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He has perfect chakra control however(like Sakura) and thanks to this, he can perform the technique requiring 30% of his chakra without problems. But because there would still be a 10% of chakra left and
you can't reconvert chakra back to stamina, it would be a waste of 10% of chakra in the end
In Naruto's case it was even worse. Because not only does he convert stamina to chakra even worse than Sasuke(50% of his stamina into chakra when 30% is only needed for example), but he also has a poor chakra control, so if for a technique he needs 30% of the converted chakra(50%), he could only use 10% of it and thus accusing a 40% of chakra lost with the inability to reconvert the built chakra back to stamina
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