An interesting pawn sacrifice that, while not entirely correct (meaning it doesn't give White an advantage if Black accepts it), has some practical value. I remember analyzing these positions extensively about 20 years ago with my coach at the time, GM Gilberto Milos. The computers, of course, found new attacking ideas for White.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... f8b4  4. e2e3  4... b7b6 This is already the second Game of the Day from the Olympiad featuring this variation of the Nimzo-Indian. 5. f1d3  5... c8b7 