I guess the idea is to freeze white's queenside majority from advancing.
b3 would become what's called a "backwards pawn" -- if you're not familiar with that term you can wiki or google it... but yeah, basically the pawn physically blocks a5, and it controls b4.
The move stops the a- and b-pawn from ever moving again. And since the c-pawn already couldnt move because of the rook, you have stopped the whole pawn-avalanche in 1 move. Though it is strange that you should hang your own knight to play that move
The move stops the a- and b-pawn from ever moving again. And since the c-pawn already couldnt move because of the rook, you have stopped the whole pawn-avalanche in 1 move. Though it is strange that you should hang your own knight to play that move
I haven't looked with an engine or anything, but if BxN then Re5 seems to win the material back... and then it would be two knights vs two long range pieces (bishop and rook) which is generally pretty bad in any endgame (bad for the knights).
Think of it this way. On a5, your one pawn is stopping two of their pawns (a4 and b3). It's punching well above its weight, you might say. If you play a5, they only have one passed pawn to work with, and even if they manage a trade by protecting the b4 square and pushing b3-b4, they'll be unconnected passed pawns, and therefore weaker. On a7, it not holding back two pawns. They can (eventually) push with the b-pawn and create two connected passed pawns.
On the other hand, the reason to hesitate would be if, because it's further advanced (and can no longer go forward to get out of the way if attacked, your a-pawn becomes more difficult to defend. But that doesn't seem like a huge problem here.
Also the a pawn "fixes" the other a and b pawns on light squares since white has a light bishop.
In a game I played, the same "best move" kept popping up a few times and I'm not sure why.