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d c b 8 a Why it works
It looks like White wins a pawn: after 5.cxd5 exd5 the d5 pawn is attacked by the knight on c3, and the bishop on g5 pins the f6 knight to the queen on d8. 6.Nxd5?? looks like a clean material win — the 'pinned' knight cannot take. It takes anyway: 6...Nxd5! White collects the queen with 7.Bxd8 (no other capture is left), and then comes the in-between check 7...Bb4+. There is nothing to block with: the knight from b1 died on d5, so 8.Qd2 is the only legal move on the board. After 8...Bxd2+ 9.Kxd2 Kxd8 Black picks up the bishop and stays a knight for a pawn ahead, with the white king stuck on d2. A classic zwischenzug that breaks 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.