In mindfulness of the mind we come to know the “tides and wave patterns” of our thoughts and emotions. There is a good introduction to this practice in an article by Stephanie Noble, that relates it to being mindful of the body and mindful of the pleasant, unpleasant, and neutral aspect of each moment. You can find that article here: Third Foundation of Mindufulness – Awareness of Mental Phenomena
A catalog of talks, guided meditations, and other resources for the third foundation of mindfulness can be found here: Common Ground Meditation: Mindfulness of the Mind