The theory is that mind control can be used to trigger certain people who have been indoctrinated by programs such as MK Ultra. MK Ultra is not a conspiracy theory it is a conspiracy fact and is well evidenced in the U.S.Congressional records. One of the most common theories regarding mind control is that there is an army of Manchurian Candidates waiting to be triggered by a word or phrase, their mission is to attack specific targets on command. Another theory is that mass killings perpetrated by lone gunmen with no apparent motive are a result of mind controlled subjects who have been triggered by their handlers.
Mind control (also known as brainwashing, reeducation, coercive persuasion, thought control, or thought reform) is a theoretical indoctrination process which results in "an impairment of autonomy, an inability to think independently, and a disruption of beliefs and affiliations. In this context, brainwashing refers to the involuntary reeducation of basic beliefs and values".
Theories of brainwashing and of mind control were originally developed to explain how totalitarian regimes appeared to systematically indoctrinate prisoners of war through propaganda and torture techniques. These theories were later expanded and modified by psychologists including Margaret Singer and Philip Zimbardo to explain a wider range of phenomena, especially conversions to new religious movements (NRMs). The suggestion that NRMs use mind control techniques has resulted in scientific and legal debate; with Eileen Barker, James Richardson, and other scholars, as well as legal experts, rejecting at least the popular understanding of the concept. - Source: Wikipedia