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Why it works
After 1.c4 d5 2.cxd5 Nf6 Black intends the normal gambit recapture — and 3.e4!? dares the knight to take. 3...Nxe4?? looks like winning the pawn back with tempo-free ease, but the knight lands on an undefended square while the black king still sits on e8: 4.Qa4+! is a double attack along the rank and the diagonal at once. Any block (4...Bd7, 4...Nd7, 4...c6) leaves 5.Qxe4 picking up the piece. The d5-pawn was the bait; e4 was the hook.
Refutation
3...c6! is the standard gambit response: hit d5 before touching e4. After 4.dxc6 Nxc6 Black's lead in development fully covers the pawn (engine −0.6 for White is optimistic bookkeeping — practical equality). The e4-pawn can be considered later, once ...Bd7 or ...Nbd7 has taken the sting out of Qa4+.