The premise is simple: the company of those stably in truth will, given time, steady truth in oneself. In practice, satsang often takes the form of a teacher sitting with students, sometimes answering questions, sometimes in silence.
The Modern Non-Dual movement has adopted the form widely. Its origins are much older — Ramana Maharshi and Nisargadatta Maharaj both taught largely through satsang, often with long stretches of silence. The content, such as it was, was the space they held.