In this response, black develops a piece without committing to a pawn opening.

White normally completes the fianchetto through 2. Bb2, and black sets up the same pattern by attempting 2...g6 (where the knight shields the pawn.) White may then send out the kings knight with 3.Nf3, or claim the center with 3.e4 (with the knight being pinned).

Less common is 2.Nf3, which lets black setup a fianchetto of his own.

FICS Games Database shows the most common responses for white are Bb2 (64%), Nf3 (23%) and e3 (8%).

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Wikipedia has related information at Nimzovich-Larsen Attack.