Meditation in Buddhism is broadly divided into "samatha" and "vipassana", and these two together are called "samatha vipassana."
Samatha is a meditation that focuses consciousness on a single subject and calms the mind. Dhyana.
Vipassana is a meditation that brings awareness to the sensations that move towards the subject. Wisdom.
Part of vipassana is mindfulness, a meditation in which a specific thing is imagined in your mind. In Esoteric Buddhism, there is "Ajikan" meditation in which the letter “A” of Sanskrit is meditated upon, as well as “Gachirinkan” meditation in which a full moon-like white disc is meditated upon.
◎ Mindfulness as a Ritual of Tantraism (Sadhana, Siddha)
In the era of “Charya Tantra” when Tantric Buddhism was established, emphasis was placed on memorial services for pagodas and Buddha statues, and the content of these rituals diversified. One of the practices that emerged from this diversification was mindfulness (Sadhana, Siddha).
Practicing this tantra involves imaginative meditation, a.k.a mindfulness, in which the mind is focused on the meditative subject and the resulting image is absorbed into the mind and body.
This kind of mindfulness is a part of the practice of giving a memorial service to Buddhas and other gods, and can be said to be a visionary practice of “holiness manifestation,” as Buddha and Bodhisattva appear to exist in physical form.
◎ Meditation using Mandalas
In addition to the aforementioned “Ajikan” and “Gachirinkan” as specific forms of mindfulness, mindfulness using Mandalas is also frequently practiced.
Practicing this meditation involves placing a Mandala on a wall and sitting in front of it. After firstly looking at the Mandala quietly, close your eyes to paint the Mandala in your mind, observe the Buddhas and Gods painted in the Mandala, and become one with it. Eventually, you will arrive to a space where you are no longer aware that you are one.
Essentially, the Mandala can be said to be an important device and effective tool in effectively performing the rituals of Tantric Buddhism, as well as in the practice of Tantra.
◎ The Importance of “Vision” in Tantra Practices
Generally, in religious ceremonies and practices, auditory elements such as sounds, rhythms, and music are often used to help guide participants to their destinations, however, similarly, sound and rhythm play a big role in Tantraism.
Furthermore, not only in Tantric Buddhism, but also in rituals and practices of Tantraism, in addition to auditory elements, the “visual” sense is very effective among the various senses, as visually perceiving figures and images, such as mandalas, is important. This is not a characteristic of religion in general, but rather a characteristic of Tantraism.
Mandalas are often used as an esoteric tool because participants and practitioners are able to make the shape of Buddha and other Gods manifest before your eyes, because of the visual sense’s inherit power of perception and the ability to thus clearly imagine the picture, making rituals and practices, such as meditation using Mandalas, possible.
Reference: Mandala kanso to mikkyo shiso (Author: Musashi Tachikawa. Shunjusha