I was reading Tal-Botvinnik 1960 and came across a term I had not heard of before. Tal says in the book, in exact words, "Why doesn't black challenge the move a4". For context, this was about Game 2 in which Tal explores a variation where White goes a4 to stop b5. Does the term "challenge" mean to just go b5 anyway?
challenge means to "just go b5 anyway" when considered tactically, but this misses the ideas.
Chess is a game of who has the better idea. When deciding on a move will you further your own idea or challenge your opponents idea?
The game is a benoni structure where Black may try to expand rapidly on the queenside with great effect, ..a6, ..b5 etc. gaining space while driving back White's pieces.
White's a4 counters this idea, trying to suppress queenside expansion with the idea of a further a5
Challenging a4 means "You cannot stop me, I will expand on the queenside anyway!" this doesn't necessarily mean that ..b5 is the next move.
I didn't find Tal's sidelines very illuminating. Try this amazing game which features a battle of whether Black can expand on the queenside, with a thrilling conclusion.
[Event "European Club Cup"] [Site "Ohrid MKD"] [Date "2009.10.08"] [Round "5"] [White "Maxim Rodshtein"] [Black "Vugar Gashimov"] [Result "0-1"] [BlackElo "2740"] [ECO "A62"] [EventDate "2009.10.04"] [WhiteElo "2623"] [FEN ""] 1.d4 Nf6 2.c4 e6 3.Nf3 c5 4.d5 d6 5.Nc3 exd5 6.cxd5 g6 7.g3 Bg7 8.Bg2 O-O 9.O-O Re8 10.Bf4 a6 11.a4 b6 12.Re1 Nh5 13.Bg5 Qc7 14.e4 Nd7 15.Rc1 h6 16.Bd2 Qb8 17.Bf1 Ra7 18.b3 Nf8 19.h3 Rae7 20.Qc2 g5 21.Kg2 Ng6 22.Bd3 Nf6 23.Rh1 Qb7 24.Rce1 Bd7 25.Bc1 b5 26.axb5 axb5 27.Bb2 c4 28.bxc4 bxc4 29.Bxc4 Bxh3+ 30.Kg1 Rc8 31.Ba2 Nxe4 32.Rxh3 Nxc3 33.Rxe7 Nxe7 34.Bxc3 Rxc3 35.Qe4 g4 36.Qxg4 Qa6 0-1