Lesson
White to move. Main line: 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Nbd7 5. cxd5 exd5 6. Nxd5 Nxd5 7. Bxd8 Bb4+ 8. Qd2 Bxd2+ 9. Kxd2 Kxd8
After 6.Nxd5 Nxd5 White thinks 7.Bxd8 simply wins the queen — but 7...Bb4+! is the point. The bishop check must be answered, and by the time White untangles with Qd2 Bxd2+ Kxd2, Black recaptures on d8 with the king a whole piece ahead.
The trap works because the d7-knight defends the f6-knight: when White grabs on d5 twice, the recapture ...Nxd5 leaves the queen defended and sets up the ...Bb4+ fork of king and (via the pin) the bishop on d8. Develop the b8-knight to d7 in the QGD and this resource is always in the position.