This is one variation from Bird's Opening:
- f4 e5 2. e4 exf4 3. Nf3 d6 4. d4 g5 5. h4 g4
The most popular move is Ng1 instead of Ng5. Am wondering why retreating the knight is a good idea. Doesn't this move violate some opening principles in developing your pieces? After white retreats the knight, he has only three pawns developed while black can continue with Bh6, developing the bishop whilst supporting the f4 pawn.
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