[%c_effect b5;square;b5;type;Inaccuracy;persistent;true] This looks very clever, but actually, there's a better move.
1. c2c4  1... g8f6  2. g2g3  2... e7e6  3. f1g2  3... d7d5  4. g1f3  4... d5d4 One of the most aggressive options for Black, deviating from the solid play of the Catalan Opening and leading to a more unbalanced game, characteristic of the Benoni Defense pawn structure (here with reversed colors). 5. e1g1  5... c7c5  6. e2e3  6... b8c6  7. d2d3  7... f8e7  8. e3d4  8... c5d4  9. f1e1  9... e8g8  10. b1a3 [%c_arrow a3c2;keyPressed;none;from;a3;opacity;0.8;to;c2;persistent;false,a2a3;keyPressed;none;from;a2;opacity;0.8;to;a3;persistent;false,b2b4;keyPressed;none;from;b2;opacity;0.8;to;b4;persistent;false] A classic plan: the knight goes to c2, and White will try to play the b2-b4 advance at some point. 10... h7h6 [%c_arrow e7c5;keyPressed;none;from;e7;opacity;0.8;to;c5;persistent;false,e7d6;keyPressed;alt;from;e7;opacity;0.8;to;d6;persistent;false,e6e5;keyPressed;alt;from;e6;opacity;0.8;to;e5;persistent;false][%c_highlight c8;keyPressed;none;opacity;0.8;square;c8;persistent;false] A subtle and difficult-to-understand move. I believe the idea is to move the bishop from e7, maybe to c5 (as in the game) or to d6, supporting the e6-e5 advance, which opens the diagonal for the bishop on c8. The pawn move is useful to guard against Bg5 after the e7-bishop moves. 11. c1f4  11... e7c5 [%c_highlight d4;keyPressed;none;opacity;0.8;square;d4;persistent;false] This is a new move. The bishop is in a firm position on c5, protecting the d4-pawn. The problem is that White will prepare b2-b4, attacking the bishop, but Black can prepare e6-e5 and return the favor. 12. a3c2  12... f6d7 [%c_effect d7;square;d7;type;GreatFind;persistent;true] 13. a2a3  13... a7a5  14. b2b3 Necessary to prevent a5-a4. 14... f8e8 Both sides try to execute their pawn advances, White on b4 and Black on e5. 15. f4d2  15... e6e5  16. b3b4  16... c5d6  17. c4c5 [%c_highlight d6;keyPressed;none;opacity;0.8;square;d6;persistent;false] So far, both sides have played well, and the position is balanced. This is a critical position. Where should the bishop go $2 17... d6f8 [%c_effect f8;square;f8;type;Inaccuracy;persistent;true] The wrong square. 18. a1c1 [%c_highlight e5;keyPressed;none;opacity;0.8;square;e5;persistent;false] The position is unpleasant for Black now since the e5-pawn is a target, and his pieces are underdeveloped. 18... a5b4  19. a3b4  19... a8b8 [%c_effect b8;square;b8;type;Mistake;persistent;true] After this mistake, it's really hard to defend. 20. c2a3 [%c_effect a3;square;a3;type;GreatFind;persistent;true][%c_arrow a3c4;keyPressed;none;from;a3;opacity;0.8;to;c4;persistent;false] The knight takes advantage of the free space and prepares to go to c4. 20... b7b5 A desperate attempt to confuse the game.