Whenever the word "mind" pops up in a Buddhist text, it's usually a good idea to find out which mind it is talking about. In the Pali Tipitika, for example, there are three words commonly translated as "mind," and they all mean different things. Manas is associated with judgments and volition, and vinnana is the mind of ideas and cognition.
But the third foundation is cittasati, or mindfulness of citta. Citta is the mind of mental states or qualities -- sleepiness or alertness, for example. Citta is sometimes translated "heart-mind." At its most basic, this is mindfulness of states of mind. Through dispassionate observation, we see how ephemeral they are.