I am watching some commentary on the World Championship Games. This is from Game 1:
[FEN "rnbq1rk1/ppp1ppbp/6p1/8/3PP3/2B5/PP1Q1PPP/R3KBNR b KQ - 0 8"] [White "Anand"] [Black "Carlsen"]
The commentator states that Carlsen has several moves available to him at this point, including one which confuses me: 1. ...Bg4. I see that it stops White from castling, but after 2. f3 that Bishop seems silly. The pawn structure looks unfavorable but not fatally so after a bishop retreat. Alternatively, White can also play 2. Nf3, 2. Ne2 or 2. Be2 to set up an exchange.
What's the deal with 1. ...Bg4?