The Knight Trap comes from a variation of the Alekhine Defense.
1. e4, Nf6
2. e5, Nd5
3. c4, Nb6
4. d4, Nc6 ?
Standard play in the Alekhine Defense is 4....d6.
5. d5!, Nxe5
6. c5!!, Nc4
7. f4! ....
Whichever piece Black moves, he loses a Knight.
But White must not be too greedy to grab a Knight. If 7.....e6, then White must answer 8. g3 before capturing the e5 Knight in order to avoid the devastating reply 8....Qh4+.