[mshalen]

The book is discussing how to play against a loose-passive player. I think that the point he trying to get across is that if a player, who is loose-passive, will usually check-call with most hands but suddenly makes an out of character move (here that would be raising on 5th when catching a seemly useless card) then ( to paraphrase Sherlock Holmes) no matter how improbable the original holding may be, when they make a bizare move, then they will typically be holding the improbable hand they are representing.

Haven't all of us seen strange moves and said to ourselves "the only hand that could make that bet would be X, and no one in their right mind would have played those cards on that board"? Then they flip over the exact cards you thought they couldn't possibly have.