comment: There's a small detail White has to keep in mind when choosing this instead of 9.exd4. In 1999, I played 9.a3 with White and thought I was very clever doing so, but fifteen minutes later, I wasn't sure anymore. Actually, I'm still not sure about it 25 years later.
context: 1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... f8b4  4. e2e3  4... b7b6 Black has many options in this classic Nimzo-Indian line. 5. f1d3  5... c8b7  6. g1f3  6... e8g8  7. e1g1  7... c7c5  8. c3a4 This move highlights the problem of not capturing the knight earlier. If you play similar positions with Black, always watch out when White castles and gets rid of the knight pin. If you forget about it, you might find yourself with a trapped bishop. 8... c5d4 The bishop prepares to come back.