We're talking about a quite advanced tribe this time, which even has a spiritual system and temples where religious practices can be done.
In one of these temples there's an ancient tradition of training girls to be flexible for a special purpose. In the past these specially trained girls were used as "demon guardians" of the temple, the living statues whose inhuman positions and dreadful masks successfully scared off the unwanted people, for example undesired guests from other tribes or thieves who wanted to steal from the temple.
The demon guardians were also very good actors, they normally pretended to be calm, but when a special sign was given, they would start to "awaken", hiss loudly and move in fearsome ways. For example, when a guest visited the temple and started talking rubbish about religion, the priest would give the "demonic" girls a sign and they started to "awaken" which would usually frighten and convince the guest and may even convert him. So the girls served well to keep the temple very peaceful this way.
In the present day, many centuries later, the demon guardians can still be found in this temple, and some very religious people are scared of them even now, but for most people they serve a purely decorative and entertaining purpose. They dance quite beautifully during celebrations and sit still during the usual days, staying as grim but beautiful living statues.
Some people, however, consider them bringers of luck and make daily offerings to these living statues. Usually the offerings to the demon guardians are made in the form of coins, the bigger coin the better. So needless to say that the speciality of the demon guardian has become quite popular and many girls now strive to get this position, so to speak. But only the best few of them, the most flexible and the most artistic ones, are chosen to work as the demon guardians.