1… Nc4 takes advantage of the pinned pawn on b3 and in effect traps the white queen.
Here’s the variations that show how black wins material:
- 1… Nc4 bxc4 2. Qxa3 or
- 1… Nc4 2. Bd3 Nxa3 3. Rxc3 Rxc3 or
- 1… Nc4 2. Re3 Nxa3 3. Rxc3 Rxc3
Note that 1… Nc4 clears the b6-square, which means the black pawn on a6 is now defended by the rook on c6.