[1] It is widely used in psychophysiological studies on fear and phobias that investigate the preattentive nonconscious reactions to fear-relevant stimuli.
However, one theory for this phenomenon, known as the dual channel interaction theory, proposes that a fast signal created by the second stimulus is able to catch up to and overcome a slower signal sent from the first impulse.
While not consciously perceived, the masked stimulus can nevertheless still have an effect on cognitive processes such as context interpretation.
It has been shown that visually masked stimuli can elicit motor responses in simple reaction-time tasks (e.g. response priming) independent of their conscious visibility.
[4] It is a widespread belief that masked stimuli can be used for psychological manipulation (see subliminal messages, psychorama).