Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...Nd5

= Two Knights Defence =

4...Nd5?
Rather than blocking with the pawn, black elects to use the knight. This opens a discovered attack on the white knight as well.

White should not agree to trade knights with exd5 or Bxd5. This will give Black counterplay with their queen on g5 and force White to castle, completely losing their attack on Black's f-pawn.

Theory table
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 Nd5