When one move makes the difference

One move can make a difference. It looks obvious, but I have two different meanings of it. The first is that a move in a game can change the winning side, and the second is, when there is a race situation (for example, a race for promotion at end games), being advanced by just one move in the race can make the difference too.

Let's see this situation from a real game (I was playing black).

This was a clear situation of white win and my opponent playing white decided to get both rooks out of the game to translate the situation to win.

After 1. Rxc5+ Kxc5, he played 2. axb5 and he was expecting me to play 2. ... axb5. And the game will be in a situation that he has 3 pawns while I have only 1. His king will go to the queenside of the board to stop me from promoting while I'll have 2 options. Either to push my king in the queenside trying to protect my pawn and in the meantime he will promote one of his kingside pawns or I may push my king to the kingside in a hopless try to prevent his 3 pawns from promotion and in the meantime he can capture my pawn in the queenside and come back to join the battle in the kingside to secure promotion of one of his pawns. Seems a hopless situation for me. Isn't it?

But wait a minute, Let's have another look! After capturing his pawn I'd need 5 moves to promote while he needs exactly the same number of moves to put his king in the square where I need to promote and end my hopes. Trying to quickly run with my king to secure the promotion square before his king gets there can succeed, but it will give him the time he needs to promote too and the game will be a win for him after trading both queens and promotion of one of his remaining 2 pawns or a draw in the best case for me.

A question came to my mind, do I really need to capture his pawn at b5? what about just playing a5 without capturing his pawn? This will mean that the promotion battle is going to take place on the "a" file instead of "b" which means he will need one more move to prevent me from promoting. So, I'll need 5 moves to promote while he needs 6 moves to prevent me and I have the move now which means his king will be 2 squares away of my pawn at the move of promotion which is all what I need to securely promote. Also his pawn at "b" file isn't a problem because my king will be able to capture it once he tries to push it forward and this will not affect the race as it will take one move from each of us. This one move (a5 instead of axb5), makes the difference!

So what if he decided not to try to prevent me from promoting and instead push one of his pawns to promotion? He'd need 5 moves to promote any of his pawns, the same number I need. So, will we both promote at the same time? No! with my promotion, I am going to give him a check and he will need to consume a move to escape my check which is enough for my just promoted queen to revert back and prevent his advanced pawn from being promoted. Again, one move made the difference!

Needless to say that after few moves, he resigned!