What checks does White’s bishop have? Bxe6. Does the move attack anything else? Yes, the bishop at d7, which is an unsuitable target. Or see it visually: we have a classic triangle between Black’s king, queen, and e6 pawn; the bishop can be understood as an inadequate target or as an obstruction. Either way, the natural next step for White is to capture the bishop with another piece—as with RxB. Then if Black recaptures with QxR the board is prepared for Bxe6, now with Black’s queen as the target. Like the bishop that it replaced, the queen is a problematic target because it can fight back against the forking piece; but unlike the bishop, the queen’s value is enormous. So the fork works as long as White’s bishop has protection when it delivers the fork. It does; it will be guarded by the White queen. Play therefore goes 1. RxB, QxR; 2. Bxe6+, QxB; 3. QxQ+, and now White has won a queen, a bishop, and a pawn in return for a rook and a bishop.