Do not turn to mediums, nor seek out oracles, so as to defile yourselves through them. I am YHVH your Elohim.

Rashi defines yidoni as inserting a bone of an animal whose name is yadua into ones mouth and the bone speaks. We are told not to seek or get involved with this type of practice, for if you become involved with them, you make yourself impure before YHVH. If we practice these things, we are exchanging our relationship with YHVH with idolatry... .and that is why He says, "I am YHVH your Elohim!"

Yeshayahu (Isaiah) 8:19 When they tell you, "Consult with those who have familiar spirits and with yidoni, who chirp and who mutter:" shouldn't a people consult with their Elohim? Should they consult the dead on behalf of the living?

We see that the yidoni chirps like a bird and mutters (perhaps in some type of mystical tongue). It is because of this verse that the Talmud says that practice involves using the bone of a bird. (Sanhedrin 65b). and most probably incense drugs and meditation (Yad, Avodath Kokhavim 6:2; Sefer HaMitzvoth, Negative 9; Ralbag). The Septuagint translates yedoni in this verse as proskolliethiesesthe, denoting one who seeks to open the mystical. Elsewhere, it is translated as gnostas (on 1 Samuel 28:9), one who seeks gnostic experiences.

It is interesting that the greek word for sorcery is pharmakeus, from which we get the word pharmacy. It describes a drug induced spell.

Vayikra (Leviticus) 20:6 And the soul that turneth after such as have familiar spirits, and after wizards, to go a whoring after them, I will even set my face against that soul, and will cut him off from among his people.

To be involved in this practice means that we are being unfaithful and getting involved with spiritual adultery.