1.Nf3 f6 2.Nd4

This line is known as kamikaze attack. The white knight threatens to directly go to the enemy side with 3.Nf5 or 3.Nb5.

The best reply is 2...Nh6 threatening ...Ng4 and ...Nxf2.

The tricky 2...Nc6 is countered by 3.Nf5 e6 4.e4 Nd4 5.Qh5 g6 6.Nxg7.

After 2...e6 3.Nb5 Nh6 black has some traps but white is winning, for instance 4.f4 Ng4 5.e3 and black has nothing.