Foreign keys

I have general question about FOREIGN KEYs:

1. Suppose I have table A with primary key X, and another table B with field Y.
2. When I 'ALTER TABLE "B"  ADD FOREIGN KEY( "Y" )  REFERENCES "A"  ON UPDATE CASCADE  ON DELETE CASCADE', that clearly spends some time building a separate index.  Since there is already a unique index on X, presumably (?) the index being built is on Y.
3. However, the PostgreSQL documentation seems to indicate that it's a good idea to also separately create an index on Y.
4. Why, and why is the FOREIGN KEY index different from the ones on X and Y in any way but trivial?
5. If I need the separate index on Y, should it be built before or after the FOREIGN KEY constraint?
   

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