Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e6/3. Nc3/3...Bb4/4. Bg5/4...h6

= Nimzo-Indian Defence, Leningrad Variation =

4...h6
Black drives the bishop back, so as to gain the ability to play ...g5 and break the pin at a later time. 5. Bxf6 Bxc3 6. bxc3 Qxf6 gives up the bishop pair and lessens White's ability to defend the weak c pawns, especially once they become fixed in place with ...c5, so the only good move is 5. Bh4.