His release from the posthypnotic suggestion had made it possible.
A special code phrase in the message had released "Stanley Martin" from the posthypnotic suggestion that had held him for so long.
Using this technique, with each subject carrying out a posthypnotic suggestion, intensifies the responses of other subjects.
That apparitions of a sort can be induced by hypnotic and posthypnotic suggestion is certain enough (see Hypnotism).
posthypnotic suggestion n.
A suggestion made to a hypnotized person that specifies an action to be performed after awakening, often in response to a cue.