A nganga

refers to a certain creation made with an iron cauldron into which several items (such as bones and sticks) are placed. It also refers to the spirit of the dead that resides there. In Palo, it refers to an iron cauldron used to imprison evil spirits using chains, padlocks, knives, etc., which can be used for black magic.
In the following video it can be appreciated how it is fed. Feeding the Nganga depends on several things; what nganga it is, what spell is being done, etc. They are usually fed with the blood of animals, the most commonly uses are chicken (roosters), pigeons and goats. Feeding them also involves certain chants and spells. Also what time of the day are they fed and for how long. This practice also involves rum and cigars.
If a spell is going to work depends greatly if the spirits are happy and fed. In the contrary, if a nganga is not fed and taken care of, not only is the spells not going to work, but the spirits will go against you. The relationship between and nganga and the voodoo priest is one of mutual respect, more of a partnership than that of an employee/employer. Similar to a normal person, nganga like some things, and dislike others. The creation of one is something complicated, which requires a good array of ingredients, both material and spiritual.
Feeding a nganga is one of the many things a voodoo spell requires. Many preparations are done before the spell is being casted. The more information is at hand before the voodoo spell is done the better chances the spell has of working and of taking action faster. Although very similar, spells vary according to the branch of the voodoo they belong too. The similarities like in the ties with the spirits the voodoo doctor has, and the handling of nature and the many material this provides. Herbs are a very important part of it, using the right herbs is as important as using the right spell for the ritual. This is usually found naturally around Latin America and the Caribbean. Doc Buzzard is a very experienced voodoo shaman whom has many nganga at his disposal for all kinds of voodoo spells.
All in all, the relationship between the Voodoo Priest and the Nganga is one of a mutual respect and benefit, a long time relationship, which usually last a lifetime. The Voodoo doctor ask the nganga for favors and in turn provides and feeds the needs of the nganga. This relationship involves much more, and constant interaction. That is a reason why is not meant for most people, since most person would only use them instead having a relationship with them, and that is bound to end wrongly.