Shayegan, M. Rahim. 2012. Aspects of History and Epic in Ancient Iran: From Gaumāta to Wahnām. Hellenic Studies Series 52. Washington, DC: Center for Hellenic Studies. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_ShayeganM.Aspects_of_History_and_Epic_in_Ancient_Iran.2012.
Chapter 3. The Concept and Reality of the Substitute Kingin Mesopotamia and Iran
The Origins and Importance of the Substitute King Phenomenon
Nonetheless, the fact that more than one-third of the substitute’s entourage could consist of bodyguards (ša qurbūti)  indicates that despite the illusory nature of his power, the substitute was watched closely. 
Traces of the Substitute King Ritual in Achaemenid Iran
The Bearing of the Substitute King Concept on the Bisotun Narrative
In the light of this information, the relevance of the substitute king ritual for Darius’ literary subterfuge, which was intended to mask the reality of his own coup d’état against Bardiya and Gaumāta, becomes apparent. In Darius’ account it is presumed that Cambyses, threatened by an omen predicting the loss of his sovereignty to his brother, ordered his assassination and replaced him with a substitute, who by assuming power indeed fulfilled the promise of the omen. Thus, in our context, far from deflecting the omen from himself, Cambyses is accused of countering it by replacing the hostile Bardiya with a friendly substitute, a substitute who, following the death of Cambyses, became sole ruler of the Persian empire.