Duties Of A Shinigami
A Shinigami has three main duties. If a Shinigami does not do these duties, he/she will be stripped of his/her powers and may even face execution. These three duties are:
1. To perform "Soul Burials", meaning to send ghosts to "Soul Society".
A shinigami performs a Soul burial by marking the forhead of a good soul with the butt of their zanpakutou in normal form. This mark opens a gate to Soul Society and forces the soul to the other side.
2. To defeat and cleanse hollows:
Shinigami use their zanpakutous, or soul cutters to slay hollows and protect the humans and wandering souls. Contrary to some beliefs, Slaying a hollow with a zanpakutou does not destroy the hollows. Instead it purifies them so that they can journey over to soul society if they were a good in life, or to Hell if they were evil in life.
3. To balance the ammount of souls in both worlds:
Shinigami have multiple ways to balance the amount of souls. One way is to allow spirits to wander the earth for some time in order to stall for time.
Shinigami also control the amount of souls that are allowed to enter the human world through birth. That is why there are miscarrages and births. If there are already to many souls in the human world then there is a miscairrage so no soul is sent. If there are too many souls in Soul Society, there is a birth.
By balancing the number of births with the numbers of deaths, and allowing some souls to stay in the human world, shinigami keep the balance. Thusly, be bringing in more births than deaths, they bring more spirits into the human world. By lowering the amount of births to less than that of the deaths, they bring more spirits into Soul Society than than the human world. Meanwhile useing spirits in the human world are used as a temporary solution until a soul can be brought over through birth.