Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O/5...Be7/6. Re1/6...b5/7. Bb3/7...O-O/8. a4

Closed Ruy Lopez, Kasparov (anti-Marshall) Attack
Black's b5-pawn has come under attack, and its defence by the a6-pawn is only illusory as that pawn will be pinned along the a-file after axb5. Black must take other steps to defend it. Capturing on a4 would be an undesirable way out, because Bxa4 in response would renew the pressure on the c6-knight, and by extension the e5-pawn, that Black already went to the trouble of removing with 6...b5.
 * 8...Bb7 defends the pawn indirectly by defending the a8-rook, thus cutting out the a-file pin, while also developing Black's last minor piece.
 * 8...b4 removes the pawn from the danger square, closing the queenside and taking away the c3 square from White's b1-knight.

8...b4 has scored better in high-level games than 8...Bb7, so Black seems to benefit from retaining the option of developing the c8-bishop along the h3-c8 diagonal, and also from taking the Nc3 option away from White - a move with the side-effect of threatening the b5-pawn again, thus provoking Black into playing ...b4 anyway. As usual in the Closed Ruy Lopez, getting pieces off the back rank (which 8...Bb7 would achieve) is not either side's top priority, especially for Black who is slightly ahead in development. Now every pawn advance gives up squares along with gaining squares, and 8...b4 does give White the extra option of sending the b1-knight to c4 via d2, from whence it can hop across to its favourite square e3 and eventually f5 - but it has a perfectly good route to e3 already, via d2 and f1.

A sideline is 8...Rb8, which directly defends the pawn but at the cost of abandoning the a-file to White.