Did this lesson today and wow it helped me to see a pass pawn promotion situation much quicker. Say the game is down to just 2 kings and 1 pawn, how to quickly know if you can get your pawn promoted or as the chasing king if you can capture the pawn. This concept 'Rule of the square' makes it so dang easy to quickly know, maybe your opponent will not know this and you can promote even easier. This is a 'end game' technique, BTW Chess.com is made in PHP. Check out the link.