To apply it, picture the square whose side is the distance from the pawn to its queening square. If it is the defender's move and the king can enter that square, the king will reach the pawn in time; if it cannot, the pawn promotes. Remember that a pawn still on its starting square may advance two squares, so count from the square it can reach.
This shortcut is indispensable in king-and-pawn races and time scrambles, letting you evaluate passed pawns at a glance instead of calculating. It is one of the first pieces of endgame knowledge every improving player should internalize, alongside the opposition and key squares.