RELAXED ALERTNESS, ALERT EASE, PLIANCY    [shin sbyangs]
What is alertness? It is the pliability of body and mind in order to interrupt the continuity of the feeling of sluggishness in body and mind. Its function is to do away with all obscurations.
-Abhidharmasamuccaya
Alertness is an awareness in which the mind is made to serve the positive as a docile servant serves his master. It interrupts the contintuity of the feeling of sluggishness in body and mind.

Alertness is twofold:

1.    Physical alertness
2.    Mental alertness
Physical alertness means that when through the power of concentration the sluggishness of the body, which does not allow one to do anything, has been overcome, one feels light like cotton floating in the air and the body can be made to work towards any positive value one wishes. Mental alertness means that when through the power of concentration, mental sluggishness has been removed, the mind moves on towards its object without friction and can operate smoothly. It is as the venerable teacher (Tsong-kha-pa) states,
Concentration is the king that rules the mind.
When he is seated, he is immovable like Mount Meru.
If he travels, he goes to all positive values
And brings about great happiness which consists in the pliability of body and mind.
The statement here that its function is to do away with all obscurations means that, through the power of alertness, sluggishness of body and mind is cleansed, and, when one has alertness, one is drawn towards integration from within. This integration through its mere spreading increases the feeling of pleasure, and by its mere increase integration becomes ever more intense, and so one becomes powerful to do away with all obscurations.