I don't know the meaning of some preposition:
Johny Hendricks unleashed a powerful left hook that landed right on Jon Fitch's chin, knocking down the former UFC Welterweight title challenger senseless on the canvas. The referee Steve Mazzagatti rushed in to stop the action, awarding Hendricks the biggest victory of his MMA career.
What does 'in' referred to in the phrase 'Mazzagatti rushed in to stop the action'? The referee was already in the ring/cage with the fighters, and there was no other enclosure for him to physically go into.
He intervened between the fighters.
He put himself in between them.