In the thousand years of war against the Alliance of the Holy Covenant, the Imperium of True Human Beings was also faced with the problem of the citizens of the Imperium being captured and brainwashed by the Covenant Alliance. A lot of people of the Covenant Alliance as well as the previous citizens of the Imperium had been captured and brainwashed by the Covenant Alliance.
In order to recover the brainwashed citizens of the Imperium and to explore the mysteries of the "Three Fundamental Laws", the Imperium performed all kinds of research on the brains of the people of the Covenant Alliance. Precious brain tomography had been retrieved, allowing them to gain a better understanding of the brainwashing technology.
The so-called "brainwashing" looked mysterious and unbelievable, but it was in fact just an upgraded version of mind interferential technologies such as psychological hints, deep hypnotization, etc.