The Reticular Activating System has a direct role in attention, that is what we selectively direct our attention to. This is as it regulates cortical arousal by sending constant bursts of neural impulses to areas on the cerebral cortex to maintain arousal, in response to upper and lower brain areas, and thus influences when we are awake, asleep or any state on the continuum. In terms of selective attention, in reponse to information from upper and lower brain areas (i think) it can send bombardments, generally highlighting important/emergency sensory information, of neural impulses to certain areas on the cerebral cortex in order to direct your attention to specific stimuli and ignore others so you are able to deal with the potential emergency situation from important sensory information.
The thalamus, has a role in relaying sensory information to their appropriate cortical areas, except for smell.. which bypasses the thalamus. In determining what we attend to, it can highlight sensory information of importance and increase or lessen their weight or emphasis to those cortical areas of that sensory information. Thus it can have a role in determining what important sensory information we attend to and the other sensory information which does not get attended to and thus less weight or emphasis is put upon it. In relation to on and off im not sure.. maybe this is the same thing.
Hope i helped