The name was popularized by Chase, Chin, Oppezzo, and Schwartz, who studied eighth graders using Betty's Brain, a system where students teach a virtual agent that then reasons from what it was taught. Students who taught the agent spent more time on learning activities and learned more biology than students studying for themselves, and the benefit was largest for lower-achieving students.
Several mechanisms plausibly combine: a teacher must organize material coherently, anticipate questions, and monitor the pupil's understanding, which doubles as monitoring their own. There is also a motivational component, described in the research as responsibility toward the pupil, that makes failure feel consequential in a way private studying does not.
You do not need a classroom to use it. Writing a public summary of an article, explaining a paper to a colleague, or maintaining shared notes all put an audience, real or imagined, on the other side of your learning. Reading with the intention to explain changes what you notice, what you highlight, and what you bother to resolve.