The mental factor that checks the condition of mindfulness's mental hold on the object of focus. It sees if the mental hold has been lost or is too weak or too tight due to flightiness of mind or mental dullness. It is more, however, than just reflexive awareness (rang-rig) or implicit apprehension (shugs-la rtogs-pa), which merely notices what is happening with the meditation. Alertness resembles an alarm system to trigger a response with restoring attention to correct any faults.
Tibetan: ཤེས་བཞིན། shes-bzhin
J. Hopkins: Introspection