Chess Opening Theory/1. e4/1...Nf6/2. e5/2...Nd5/3. c4/3...Nf4

= Alekhine's Defence: The Squirrel =

Black saves their Knight, which was attacked by the pawn on c4. Even though the knight moves forward, this move is not seen as best because the Knight is unstable on f4 and will likely have to move again to a worse square to save itself.