The Kun Shou is the process by which a Hsien acquires a human body.
The Shinma, unlike their Western counterparts, don't displace the mortal soul within a human body or merge with it (an explanation some Kithain suggest for their rebirths. Instead the Hsien finds a dying human whose soul is fleeing. Only when the soul has departed dos the shinma seize the body for his or her own use. This ritual is the Kun Shou and the new form is called the Hotei. When the Hsien's current incarnation is destroyed they find a new host body nearby. Hirayanu seem to end up in the bodies of infants or children more than adults while the opposite is true for the Kamuii. The Kun Shou has a tendency to cloud memories of previous incarnations. Most shinma would prefer a body that has attributes they prefer. A Nyan would obviously like a young and attractive person. This isn't always possible, though, and the new form restricts the abilities of the shinma. Unborn infants can be used, young persons killed in accidents, suicides, even venerable old grandparents can be used. Over time the human shell may take on subtle aspects of the kwannon-jin within it. A shinma who inhabits an adult is called a Hsien-jin. One who inhabits a child or infant is a Hsien-tsu. The hsien-tsu doesn't become a Hsien-jin when they reach the age of 15. They remain a Hsien-tsu for the whole of their incarnation.
The plus side to al this is that sometime "miraculous recoveries" sometimes happen. The family member didn't die in the wreck, despite all probability, but they changed personality radically and left their family suddenly.
It is possible for the more callous Shinma to purposely prepare a body for habitation but no one would discuss such a foul practice openly. It does happen, however, though such wanton murder is against the Mandate of Heaven. Shu Shen, especially, are not above the practice.