The Mentalist invites two chess players on stage. These are not plants, but random people who understand something about chess. He tells the audience that, before the show, he gave a chess book to an audience member. He had them take the book to the lobby, circle a chess piece on any page, and then note the page they circled. They were then instructed to seal the book in an envelope and keep it in their possession. They were further instructed to TELL NOBODY, including the performer, which piece they circled.
The performer then calls for the sealed package. The package is then placed on a table in full view of the audience. The table also contains a fully set-up chess set. The Mentalist then asks that the first spectator go over and stand in front of the chess set blocking it from view. He is given an unprepared paper bag and asked to SECRETLY drop any chess piece in the bag. He is cautioned NOT TO LET THE AUDIENCE, or the other spectator on stage, see the piece he chose. He remains standing in front of the chessboard, blocking its view from the audience.
The performer then opens the package and has Spectator Two remove the book. The performer does not touch the book once it is removed from the package. There is a note in the package (or stuck to the book) that tells the page number of the circled piece. Spectator Two opens to that page number and shows the audience the circled piece, say, BLACK ROOK. The spectator is then asked to riffle through the book and make sure there are no other marks in the book. No other marks are seen anywhere in the book.
The Mentalist says, "The chess piece circled in the book, before the show, is the BLACK ROOK. Wouldn't it be incredible if the piece the person secretly put in the bag is also a BLACK ROOK?"
Spectator One removes the chess piece from his bag; it is shown to be a BLACK ROOK! A totally amazing coincidence that appears impossible.