Spoiler: showIt's probably always on. The jutsu they do probably just increase blood flow/chaka to the eyes as some sort of enhancement. Basically one of luffy's gears only it's for the eyes only.
Just my theory.
Also the ninjas have advanced enough technology/jutsus to clone dead people. Would it really be hard to find just one sample in an environment where blood is flying all over the place? The real challenge/problem with trying to obtain a hyuuga sample would be speed.
I'm assuming/theorizing that the seal on branch members destroys all DNA and what not once activated but until then any sample taken is viable. In addition there's probably some weird chakra field or genjutsu that makes it so that detached limbs are also made unusable(or simply once a body part dies the seal takes effect on it). The seal is the antenna and the rest of the body is tuned towards the antenna for when the kill switch is used.
In addition the hyuuga probably have main house members lurking around in battles specifically to seal off any branch members that die.
Ergo any battle vs a hyuuga where you want to steal an eye must occur with you doing so before a main house member that can see for miles away and stop you at any time is able to do so.
In addition since the body other than the actual seal is tuned into the "antenna" you must purge the actual genjutsu that causes deterioration quickly as well.
Essentially you'd have to either be one fast mother fucker or one badass guy to take out a main house member of the hyuuga(and the anbu from the villages that go around making sure you cant loot corpses).
Also the fact that the hyuuuga have so much security concerning their DNA could mean that ninjas in the past used to actively rip out the eyes of any hyuuga they got their hands on for x ray vision which leaves the possibility of "ninja cloning" of old samples to get new eyes. Of course such practices would be used only for those whom could be trusted with them and as such not very widespread. As a plus, keeping it a secret is very useful. After all it's one thing to have a hyuuga nearby when keeping secrets but it's entirely another to not have a fucking clue that there's a person able to see every last thing you're doing when you want to hide shit.
XI Wiki



