Black to move, played:
White's best would now be 2. Nxe4 d5 3. Bd3 dxe4 4. Bxe4, in which case the prospects would remain even. But before taking the Knight on e4, he decides to use his Bishop to prevent his opponent from castling - a calculation which, in this position, is faulty.
2. Bxf7+ Kxf7 3. Nxe4
At first sight White's plan appears to have succeeded, since, as well as maintaining the material balance, he has prevented Black from castling. But the further play shows that Black can carry out an 'artificial castling' without difficulty and has made a clear gain in that he is left with strong pawns in the centre.
3... d5 4. Ng3 Rf8 5. d3 Kg8
Now the superiority of Black's position is quite obvious, and one can easily appreciate the part played by the 'artifical castling'; Black has admittedly expended three moves on it, but White has derived no advantage from this fact, since he has made three moves of even less value, i.e. Bf7+, Nxe4, and Ng3. In a sense, Black was castling while White was transferring his Knight from c3 to g3.