The mechanism
Player A passes to B, and the ball is finished by C — a third player arriving from a position the defender wasn't tracking. The reason it works is attention: the defenders are watching the ball and the first two players, so the third runner slips in on the blind side. The Athletic's piece walks through lovely examples of it; go read those. The pattern is simple. The timing is the hard part.
Why I never coach it as a pattern
You can draw a third-man combination on a board in ten seconds. It means nothing until players can read when it's on and time the third run against real defenders. So I train it inside games with live pressure, not as a walk-through, because the whole value is the recognition and the timing under a defense that's trying to stop it. If I script it, players learn to run a play. If I put it in a game, they learn to see the game. I want the second thing every time.