Black signals his intention to reach a KID setup. White can choose to seize central space and transpose into a standard KID by playing 3. d4 or to stay in the English opening by playing 3. g3. This refusal to seize central space by white does give white some flexibility and tactical subtleties in return. As the usual idea in the KID is to undermine the white center the apparently dimmer 3. g3 signals that white will develop first and get some strong control over the center before actually advancing his central pawns, which will make it much harder for black to destroy them through a conventional KID strategy.

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Wikipedia has related information at English Opening.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.