White "Doubles the Knights." In so doing he offers a "Knight Sacrifice." If White accepts then 1)... gf, 2)nf5--"Knight Replacement"--qe8, 3)qg4+ qg6, 4)ne7 is a "Family Fork." Because of this threat Black's Queen must "Unguard" D6 a Critical Square and the pawn on that square and thus his whole position falls apart.
2) nd6 qe7
The Critical Pawn is now captured. And the Critical Square is now occupied.
3) nef5 gf
The "Doubled Knights" prevent the Queen from capturing the Knight and also prevents the Queen from returning to its formerly safe square. The pawn is Forced to Capture the Knight and thus provide a "Safe Seat" for the Replacement Knight.
4) nf5 qf7
The Knight is now sitting pretty in its "Safe Seat" and the Queen's "Position is Forced." Or 4)... qe8, 5)qg4+ qg6--preventing qg7 mate, 6)ne7 wins again
5)nh6+ is "Family Fork." winning the Queen
