The Rishonim relate to your question:-

(1) (a) Rashi 65a DH Reish Lakish writes that even though placing the bone of the "Yadua" animal in the person's mouth is a Ma'aseh, nevertheless the bone does not speak immediately when it is placed in the mouth. It is only later on when it is already inside the mouth, that the bone speaks on it's own. (Rashi's source for stating that the bone speaks on it's own is from the Gemara below 65b).

(b) Since the principal part of the act of the Yidoni is the fact that it speaks and because this does not occur immediately this means that the person's involvement with this is only considered "Gramma" :- the speech of the Yidoni is only an indirect result of the worshipper's action.

(c) This can be compared to another Gemara we will learn soon, be'Ezer Hash-m, in the Dafyomi below 77b at bottom of page. The subject matter there is very different, but the Halachic principle involved is the same. A murderer tied up his victim and directed a current of water onto him and the victim drowned. The person can only be executed by Beis Din for murder if the water flowed immediately onto the victim ("Ko'ach Rishon"). However if the victim was placed a bit further away from the dam which was released and the water did not fall immediately upon him but only afterwards washed him over ,then this is "Ko'ach Sheni" and is considered Gramma . It is also an act of murder, but the Sanhedrin can not convict him of the death penalty for an indirect act, but instead his life will be taken by the Law of Heaven.

(d) One learns from this Gemara that if the result of the Ma'aseh is not apparent immediately, this is considered Gramma - indirect - and does not possess the status of a Ma'aseh in Halachah. Therefore, since the bone does not speak immediately when it is placed in the mouth, this is also not considered a Ma'aseh. In addition, since when the bone does speak it speaks on its own, this also means that the speech is not directly connected to the action of the person who placed it in his mouth, and therefore the speaking is not considered as his Ma'aseh.

(2) The Rambam - according to the explanation of the Lechem Mishneh Hilchot Avodat Kochavim 6:1 DH HaOseh - has a different approach. When, in Hilchot Shegagot 1:4 the Rambam lists all the transgressions for which one brings a Korban Chatat, he writes that the 4th general (not Arayot) transgression is to do the Ma'aseh of the practitioner of Yidoni. The Rambam is stressing that one is only culpable if one does a Ma'aseh. In addition in Hilchot Avodat Kochavim 6:2 the Rambam writes that a bone from a bird [not an animal as Rashi writes - D.B.] named Yadua is placed in the practitioner's mouth and then he burns incense and performs other acts until he falls down like an epileptic suffering an attack and pronounces future prophesies. This is clearly a Ma'aseh. [Note that Rambam explains the Gemara 65b which states "and he speaks on his own" to mean that the person speaks on his own. In contrast Rashi writes that the bone speaks on its own - D.B.].

The Lechem Mishneh writes that according to the Rambam there are 2 types of Yidoni - one without a Ma'aseh and one with a Ma'aseh.

(a) There is a category of Yidoni whereby someone forecasts future events by guessing, without placing the bone in his mouth.

(b) However most people who practice Yidoni do so with a Ma'aseh in the way that the aforementioned Rambam describes. Therefore Yidoni is a general name for people who predict future happenings because it is usually done with the Yadua bone, but even type (a) that predict events without any bone but simply by normal speech, are also termed Yidoni.

Lechem Mishneh writes that when the Gemara here states that Yidoni is without a Ma'aseh, this means that Yidoni is sometimes without a Ma'aseh and therefore the Gemara in Masechet Keritut did not list it among those which must offer a Korban Chatat. However most cases of Yidoni do involve the Ma'aseh with the Yadua bone.

Here is a different answer to your question.

The act of placing the "Yadua" bone in the mouth is not a forbidden Ma'aseh as such. What is forbidden is the "Keshafim" - magic - that is performed after the bone is placed in the mouth [See Rambam that I cited above in (2) that he burns incense and performs other acts which cause the speech]. Since the act done with the Yidoni bone itself is not forbidden in its own right, the Gemara therefore refers to Yidoni as "Ein Bo Ma'aseh"

Kol Tuv

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