If the white king can occupy a key square, he can force the promotion of the pawn but accurate play is required.
Whether or not the white king can reach a key square depends on the position of the pieces and which player is to move.
If the Black king can reach any of the squares marked with a dot or an "×", it stops the pawn – either by blocking the pawn or preventing the white king from reaching a key square.
This position from a game[4] between Svetozar Gligorić and Bobby Fischer illustrates key squares.
Black to move can keep the white king from reaching a key square by 57...Kb8, so the game is drawn.
In this position, the first king to reach one of his key squares will win the opponent's pawn and protect his own.
When both kings can reach a key square, a position of mutual zugzwang may occur.
Black can prevent the white king from reaching a key square directly, for example: Taking the opposition; however, the white king can reach a key square (b5) by going on the other side of the pawn: Opposition, and Black is in zugzwang.
or and the white king has occupied a key square and has a winning position.