1) Below the surface 2) Imperceptible 3) Unperceivable 1) Hidden 2) Imperceptible 3) Intuitive 4) Involuntary 5) Kind of subtle ad technique 6) Kind of TV ad technique 7) Like underhanded advertising 8) Subliminal advertising 9) Unconscious 10) Unintentional 11) Unperceivable 
• (a.) Existing in the mind, but below the surface or threshold of consciousness; that is, existing as feeling rather than as clear ideas.
Very short commercial messages below the threshold of conscious perception that could be inserted in advertising, rules permitting.
Below the threshold of perception or excitation; below the limit or threshold of consciousness. ... Origin: sub-+ L. Limen (limin-), threshold ... (05 Mar 2000) ... 
(sәb-lim´ĭ-nәl) below the threshold of sensation or conscious awareness.
Sub·lim'i·nal adjective [ Prefix sub- + Latin limen threshold.] (Philos.) Existing in the mind, but below the surface or threshold of consciousness; that is, existing as feeling rather than as clear ideas. 
Type: Term Pronunciation: sŭb-lim′i-năl Definitions: 1. Below the threshold of perception or excitation; below the limit or threshold of consciousness.
[adj] - below the threshold of conscious perception
adjective below the threshold of conscious perception
(Lat. sub. under + limen, the threshold) Term popularized by F. Myers to describe allegedly unconscious mental processes especially sensations which lie below the threshold of consciousness. See Unconscious Mind. -- L.W.
existing or operating below the threshold of consciousness; being or employing stimuli insufficiently intense to produce a discrete sensation but often being or designed to be intense enough to influence the mental processes or the behavior of the individual: a subliminal stimulus; subliminal advertising.
[SAT terms] below the threshold of conscious perception