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d c b 8 a Why it works
It looks like White has won a pawn: after 5.cxd5 exd5 the d5 pawn is attacked by Nc3 + bishop on g5 pins Nf6 to the d8 queen. 6.Nxd5?? appears to be a clean material win — the pinned f6 knight can't capture. But the 'pinned' knight does move: 6...Nxd5! White grabs the 'undefended' queen 7.Bxd8, but Black plays the in-between check 7...Bb4+! After 8.Qd2 (the only block — Nbd2 is also possible, but Qd2 is the main line) 8...Bxd2+ 9.Kxd2 Kxd8 Black ends up a knight for a pawn ahead. A classic combination using zwischenzug to break the pin.
Refutation
6.Nf3! — calm development without going for the pawn. After 6.Nf3 c6 7.e3 White has a normal QGD position without adventures. Alternative: 6.Bxf6 Nxf6 (or gxf6) — trade the bishop, dissolve the pin, play normally.