How do Polonius, Claudius and Gertrude “employ” Ophelia in their scheme to inquire into Hamlet's motives later in the play?

They use Ophelia in an encounter with Hamlet to see his reaction to her. When she gives him the "tokens" which he once gave her, he becomes harsh in his language to her. They want to think that Hamlet's madness stems from his unrequited love for Ophelia; Claudius, however, is not convinced.