Nukitsuke and nukiuchi are different. “Tsuke” means you are acting to forestall an opponent’s attack before it begins. Nukiuchi on the other hand means, precisely, to cut down an opponent. Without understanding the difference between these two, your swordsmanship will not be effective.*
－ Kamimoto Eiichi sensei, iaido hanshi 9 dan, kendo hanshi 8 dan
This short statement highlights and clarifies an important point about iai (particularly regarding Muso Shinden ryu, Muso Jikiden Eishin ryu and ZNKR iai). It is a simple linguistic point, but even if you speak Japanese it is easy to overlook.
Kamimoto sensei clearly states that nukitsuke is not merely the act of drawing the sword and cutting your adversary. Rather, it is the application (付ける) of the sword draw (抜く) to control an opponent. This is how nukitsuke differs from simply drawing the sword and striking.
In kendo, we aim to control the opponent rather than simply strike at him. It seems – logically – that the same concept is essential in iaido as well. According to Kamimoto sensei, this “forestalling” of an attack and the controlling of the opponent is part of what defines nukitsuke – the so-called “life of iai” (居合の生命). Therefore, it follows that an understanding of the distinction described above is vital for effective iaido.
* 「抜きつけと抜き打ちはちがう。〝つける”とは相手の機先を制すということですが、抜き打ちは相手を確実に殺傷することです。その差を知っていなければ刀は生きてこないものです。」
Source: 『居合道名人伝 上巻』 池田清代著 スキージャーナル刊 ２００７年