This is a short blurb from the anime’s official website; interestingly, the wiki has nothing of the sort, but since the rest of the info on the anime website is accurate, we can assume that this is canon, as well.
The ninja soul possession happens at random and cannot be predicted, which is why it is sometimes called “Unselected Resurrection Phenomenon”.
Although it has become more common recently, the general public is not aware of that.
In some cases, the subject demonstrates the newfound ninja powers immediately after the possession, while in others, the soul goes into hiding until some sort of event awakens it (such as in Yamoto’s case).
+++ By researching ninja souls and the people possessed by them, Lee-sensei has confirmed the following basic laws (as the research is still ongoing and the sampling is insufficient, exceptions may apply). +++
① The majority of ninja souls lose their memories and ego during the possession, only retaining the karate and jitsu. An exception is made for especially powerful souls.
② A ninja soul may only possess a person of the same gender.
③ A ninja soul may only possess a living being (if the soul is forced into a dead body, a zumbie-ninja is born).
④ The person who becomes a ninja cannot have children, be it with a mortal or another ninja.
⑤ A ninja cannot become human again. If the ninja soul is extracted, the ninja dies.
⑥ During the possession, the ninja soul regenerates any wounds at tremendous rate, even able to heal mortal wounds.
⑦ The person who becomes a ninja does not age, though it is also very likely that the aging process is greatly slowed down.
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