Once freed from the body, the ancestor spirit finds itself in a strange new world. This serves to define those abilities in game terms. Spirits deviate so far from the human baseline and have so many special powers that it's simplest just to define their abilities outright. When confronted in the spirit world, these attributes define what the spirit can and can't do. In order to interact with the material world, a spirit must spend energy, expressed as a fatigue cost.
Spirits can tap into some places or people to obtain fatigue quickly.
They can use this siphoned fatigue to replenish their own store of fatigue. This energy can only be tapped into if the spirit spends the majority of the week in the area. Acts of worship from a great number of people can allow spirits to gain additional fatigue. Unlike mortals, spirits do not fatigue due to exertion, lack of sleep, hunger, and so forth. This is balanced by the fact that spirits cannot regain fatigue normally.