Learn The Fried Liver Attack
The Fried Liver Attack is and entertaining chess opening and usually Black should avoid that. We can find that opening in...
...Italian, when White is developing the bishop on c4. Then Black has two options; he can develop his own bishop on c5 or he can try the two knights defense with Nf6.
Black has the logical defensive move d5; but this is a pawn sacrifice. Black should sacrifice his pawn in order to develop his pieces quickly. In the other hand if Black captures immediately the pawn with the knight then we can have the famous Fried Liver attack.
The main idea on Fried Liver Attack is to sacrifice a knight on f7 in order to create a strong attack against the uncastled King. As you can see in the positions usually White can perform a strong attack because the enemy king is in the middle of the board.