How does Angier manage to perform Borden's New Transporting Man illusion without knowing how Borden does it?

Angier performed his version of the New Transporting Man by hiring an actor, Gerald Root, to play his double. Angier figures Borden will certainly know how Angier is performing the trick, and is frustrated that he still doesn't know how Borden is doing it. Root eventually becomes a problem and tries to blackmail Angier.