Winawer Variation
French Defense
1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5 c5 5.a3 Bxc3+ 6.bxc3 Ne7
Black pins White's knight and is willing to double White's pawns for the bishop pair and dynamic chances.
Fun fact: The Winawer, 3...Bb4, began to be played much more often after World War I, largely because of Aron Nimzowitsch and Mikhail Botvinnik. Source
Practice this lineThe idea
Black pins the knight on c3 with 3...Bb4. After 4.e5 c5 5.a3 Bxc3+ 6.bxc3 Ne7, Black has given up the dark-squared bishop to double White's c-pawns, and White has the bishop pair and more space.
It is one of the sharpest lines of the French. Black has weakened the dark squares around the king, and White's pawns are weak but cover important central squares.
Plans for White
The most direct plan is 7.Qg4, attacking the g7 pawn, which Black's missing dark-squared bishop can no longer defend. The quieter 7.Nf3 or 7.a4 keeps the bishop pair and aims for a long positional game.
Plans for Black
Put pressure on c3 and d4 with ...Qa5 or ...Qc7 and ...Nbc6. Trading the light-squared bishop with ...b6 and ...Ba6 solves the French bishop problem. Black often castles queenside, or keeps the king in the center.
Watch out for
For Black, the dark squares. After 7.Qg4, the g7 pawn and the kingside are hard to defend without the dark-squared bishop, so Black must choose between defending with ...Kf8 or ...O-O, or counterattacking with ...Qc7 and ...cxd4.
For White, the endgame. The doubled c-pawns are a real weakness, so if the position quietens down without an attack, Black is often better.