Siegbert Tarrasch gave rook endgames their most quoted law: put your rook BEHIND a passed pawn, yours or your opponent's. Behind your own pawn, the rook pushes it forward and grows stronger with every advance, since the pawn's march opens more of the file. In front of it, the rook blocks its own pawn and shrinks.
The same logic works on defense: behind an ENEMY passed pawn, your rook harasses it forever and taxes every piece that babysits it. The rule isn't absolute, nothing in chess is, but when you don't know where a rook goes in a pawn endgame, behind the passer is right so often it barely needs checking.
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White's a-pawn is passed, and Black's rook already blockades from in front. White's rook picks its post.
Follow Tarrasch's rule.
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