 Probably Black's most important idea in the Carlsbad: getting rid of the  weak light-squared bishop.
1. g1f3  1... d7d5  2. d2d4  2... g8f6  3. c2c4  3... e7e6  4. b1c3  4... f8e7  5. c4d5  5... e6d5  6. c1f4  6... c7c6  7. d1c2 