A subliminal message is a signal or message embedded in another object, designed to pass below the normal limits of perception. These messages are indiscernible by the conscious mind, but allegedly affect the subconscious or "deeper mind." Subliminal techniques have occasionally been used in advertising and propaganda; the purpose, effectiveness and frequency of such techniques is debated.
In all my years developing messaging for brands I have never had the opportunity to employ subliminal messaging as a tactic. Mostly because it is totally illegal to do so in the US, England, Australia and a slew of other countries. In the US, the Federal Communications Commission (FCC), will now revoke a company's broadcast license if the use of subliminal messages is proven. Subliminal message usage has also been banned for all members of The National Association of Broadcasters.
In a Nevada court case, the judge ruled that the First Amendment's protection of freedom of speech and press does not extend as far as subliminal messages. Because they are not speaking to a person in his or her conscious mind, but rather in a subliminal state, neither "speech" nor "press" accurately describes the nature of a subliminal message. All of these laws are steps toward protecting our vulnerable selves from subliminal messages.
That said, the English psychological illusionist and skeptic of paranormal phenomena Derren Brown demonstrates the power of this technique in the below video. Derren flips the script and manages to hoodwink a couple of advertising executives. He influences them with subliminal messaging to ideate a campaign with certain specific results. What the 'fellas come up with is exactly what Derren predicts they will.
Check out the below clip - it really pays off in the end.