Black's Knight on f6 is attacked, granting white space and a tempo with the e5 pawn push. Most often Black retreats it to d7 as Black's only 2 other viable moves, Ng8 and Ne4, either give white a significant development advantage or allow white to compromise Black's central pawn structure by exchanging Knights. This variation is one of the highest-scoring against the French, with white winning 34% of games.

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Wikipedia has related information at French Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.