Extreme effort to keep a pawn backward I'm currently studying chess strategy, and I can across an example of backward pawn exploitation that I do not understand. The book I'm reading suggests 1... Rd5 to keep the d4 pawn backward. Doesn't that put the rook into a precarious position to be attacked by 2. Bxd5? Why is it so valuable to keep the d4 pawn backward?

For reference, I'm looking at the example on pg. 236 of Silman's "Reassess Your Chess", 4th edition.

[Title "Black to move"]
[StartFlipped "1"]
[fen "3r1rk1/1pb2pp1/p1p1pq1p/2P2b2/3PBP2/PPB5/2Q3PP/4RRK1 b KQkq - 0 0"]

1...Rd5
In this case, the pawn on d4 is not just a target.  It's also restricting white's bishop on c3.  For that reason, it's very important that black not allow white to sacrifice this pawn by playing d4-d5 when white would have very good chances to hold.

Black would like to trade off light square bishops and then put a rook on d5.  The immediate 1...Bxe4 2.Qxe4 Rd5 fails tactically because white can play 3.f5! with decent counterplay on the kingside.  Additionally, at least one pair of major pieces will be traded off after white plays f5, and generally speaking the side with the isolated pawn or backwards pawn wants to trade off major pieces.

After 1...Rd5!, white cannot take on d5 because the Be4 is pinned to the white queen.  Black will play 2...Bxe4 next move and then a move like g6 in order to stop white's f5 advance.  Black will be much better - nearly winning - because black can slowly bring the dark square bishop around to f6 or g7 where it will attack the d4 pawn.  Black will also double rooks on the d file.  White, in the meantime, has no constructive plans that don't create new weaknesses.