Arnesen says Ancelotti tried something new with his formation to beat the Danes.
He revealed: "In recent times we have played with various systems, 4-3-3 and 4-4-2 with a diamond in the middle.
"For the FCK game, Carlo went with a flat middle four, which he has never tried before.
"It was a brilliant move by him because it was a hand-to-hand combat instead of a battle between two different systems. The FCK players found it difficult to compete with our midfield of Michael Essien, Ramires and Frank Lampard."