Rook Behind Passed Pawns

I'm reading a book geared toward end games.  At one point the author says get your rook behind any of your passed pawns, but does not elaborate much.  My question is: is there any place in particular I want my rook behind my passed pawn?  On the same file?  How many ranks back from the passed pawn?  Just back there anywhere, depending on the board?  - or on my 2nd rank to keep HIS rook from reaching its 7th rank?   Keeping my rook BEHIND, in general, is easier and better protection than having my rook on MY 7th rank?  How?

   Thanks for any help. 

A rook is always powerfull only from behind the pawn on the same file as your passed pawn, not from the front.

If you have a passed pawn, then exchange minor pieces and dont exchange queens or rooks

If your opponent has it, keep your minors and exchange off majors

read this:-

http://en.wikipedia.org/wiki/Tarrasch_rule

So we're saying behind AND ON THE SAME FILE as the passed pawn.  Got it.