Strategic Puzzle: explain this move

Hyia.

This is from a strategic puzzle. Black to move. The correct move is ...a5. Reason: prevent white's b3-b4.

Why? What's bad for black about b4? (Or what's good about b4?) The author doesn't explain it. I'm "puzzled" so to speak. Out of 50 puzzles, this is the only one that leaves me entirely clueless.

I think it's because b4 allows white to gain space and potentially play b5, kicking the knight and forcing it to a bad square

a5 would not have entered my brain I think

Stockfish also says ...a5 is best.

...f5 push seems like a good alternative on the surface of it, threatening ...f4, leaving the white bishop out of squares, but white can answer b4, menacing not only b5 kick, but also Bc5 skewer, winning the exchange.

It's tricky, ...a5 gains space on the flank, preventing b4 push.

But the position is tricky and complicated tactically.

I would also think first of ...a5.

Aah, now I see. Thanks to your idea that ...f5 would restrict the opponents Bishop, but I failed to see that b3-b4 would be in time to disrupt this plan.

I cannot delay by playing ...f5 first and then preventing the kick of my knight by playing ...a6. I don't have time for that because of Bc5! So ...a5 must be played right now.

Excellent @Lyudmil_Tsvetkov . Thank you.

a5 is a very important move in this position.

It is an easy move to spot when you are thinking about playing restrictive chess.

The challenge for me was that it takes 2 ply for the b-pawn to utilize its restrictive function. I thought I have time to play... a6, so what's the deal with ...a5, right? Turns out white has a simple tactical shot Bc5.

But even a bigger issue was: so what if one of my knights is pushed away? This kind of danger lurks in every game. How to prove black hasn't anything better to do than to worry about the knight being pushed away? It's easily re-routable to ...e6. Turns out there are tactical intricacies elsewhere.

For a (totally rusty) 1500 otb this wasn't an easy strategical puzzle actually. Turns out 1...a6 would do the job as well. In short: proving myself that chasing away my knight is bad for me, I feel was difficult.

The reason I would play a5 has nothing to do with Knight getting chased away or even Bc5.

The reason I would play a5 is to Restrict White.

White pawn on b3 is undefended.

It is a weakness

If white plays b4, The pawn isn’t weak because the c3 pawn will be defending it.

If black plays a5 and white responds with b4, Pawn trades would happen and white would end up with Isolated pawns.

The move a5 is basically hurting white ability to move his queen side pawns.

1 pawn shutting down 3 enemy pawns.

You can really see a difference in people's ratings with these puzzle explanations! It is interesting to hear how good players think about chess.

If the author doesn't explain things, you should probably read another book instead. Or course or whatever it is.

Not too hasty. A lot of good things happen too. As I said, out of 50 quizzes so far, this was the only one with too little explanation.

So was it "too little explanation" or "doesn't explain it"?

If there was some explanation, what is it?

The way I found a5 might be helpful.

Sometimes, you can find good moves by thinking about other moves

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When you look at Black position, you will notice Black is fully developed and is castled.

What should Black do?

The answer I came up with was I don’t know, yet.

Nothing is jumping out at me in this position.

If nothing good immediately jumps out at you, Than something in chess the best thing to do is just to make sure your pieces are defended.

In this position, Black Knight on d5 is undefended.

I don’t know what I want to do, but having my Knight on d5 defended can only be helpful.

So in this position, I started looking at ways to defend my Knight on d5.

Their are several different ways to defend Knight on d5.

I looked at several different ways to do it.

1 way jumped out at me though.

Notice the move Qe6.

Qe6 defends Knight on d5, BUT it also does something else right?

It indirectly attacks White undefended b3 pawn.

In Fact, the move Qe6 is potentially setting up a Discovered Attack on White pawn.

BUT if you play Qe6 here “Setting up the indirect Attack on b3” What will white do?

They will move the B pawn to b4 to get away from the discovered attack.

We don’t want them running away with the B pawn. We want to keep the B pawn on b3 so how do we do it?

We are thinking ahead!

At this point, we start looking at the move a5

We play a5 to prevent them from playing b4 so that our Qe6 move is even more deadly.

and all of a sudden a plan has came out of nowhere.