411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...c6/2. d4/2...d5/3. Nc3/3...dxe4/4. Nxe4/4...Bf5/5. Ng5/5...Bg6/6. N1f3
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White plays Nf3 and is about to counterattack. However, black rarely understands and notices it. There are a few popular replies to this, but black usually responds with h6, trying to kick out the knight, who's been a pain in the neck for a while.