I rewatched a few of the games where Nash was in the centre and there were a few reasons why those games coincided with better midfield performances. As a midfield unit we were drawn less to the contest, we had more people occupying the outside of the contest which didn't allow easy clearances against us, and with guys like Nash and Newcombe we also had players who provided significant on ball pressure and didn't allow the ball to leak out to more damaging opposition players on the outside.
The set-up change was the key part, that said, it was the change up in players that allowed it to happen.
Like early days Sam Mitchell who went so much better with his mate Sewell at his side