It is often broken down into two separate subcomponents that comprise this effect. The first of these components involves the self-regulation of attention so that focus is completely directed towards immediate experience, thereby quietening one's internal narrative and allowing for increased recognition of external and mental events within the present moment.   The second of these components involves adopting a particular orientation toward one’s experiences in the present moment that is characterized by a lack of judgement, curiosity, openness, and acceptance. 
Within meditation, this state of mind is deliberately practised and maintained via the conscious and manual redirection of one's awareness towards a singular point of focus for extended periods of time. However, within the context of psychoactive substance usage, this state is often spontaneously induced without any conscious effort or the need for any prior knowledge regarding meditative techniques.
Mindfulness is often accompanied by other coinciding effects such as anxiety suppression and focus enhancement. It is most commonly induced under the influence of moderate dosages of hallucinogenic compounds, such as psychedelics, dissociatives, and cannabinoids. However, it can also occur on entactogens, certain nootropics such as L-theanine, and during simultaneous doses of benzodiazepines and stimulants.
- Slagter, H. A., Davidson, R. J., & Lutz, A. (2011). Mental training as a tool in the neuroscientific study of brain and cognitive plasticity. Frontiers in human neuroscience, 5, 17. | https://doi.org/10.3389/fnhum.2011.00017
- Pagnini, F., & Philips, D. (2015). Being mindful about mindfulness. The Lancet Psychiatry, 2(4), 288-289. | https://doi.org/10.1016/s2215-0366%2815%2900041-3
- Mindfulness Training as a Clinical Intervention: A Conceptual and Empirical Review | http://www.wisebrain.org/papers/MindfulnessPsyTx.pdf
- Creswell, J. D. (2017). Mindfulness interventions. Annual review of psychology, 68, 491-516. | https://doi.org/10.1146/annurev-psych-042716-051139
- Bishop, S. R., Lau, M., Shapiro, S., Carlson, L., Anderson, N. D., Carmody, J., ... & Devins, G. (2004). Mindfulness: A proposed operational definition. Clinical psychology: Science and practice, 11(3), 230-241. | https://doi.org/10.1093/clipsy.bph077