Lesson
White to move. Main line: 1. Be4+ Kc7 2. Bxa8
Be4+ aligns bishop, king and queen in that order. Because the king is the front target, Black must respond to the check rather than save the queen. This reversal of a pin is the defining geometry of a skewer.
After ...Kc7 the b7 square is empty and the bishop’s diagonal continues through d5 and c6 to a8. Bxa8 removes the queen, leaving White with a decisive material advantage for a single developing move.
Search beyond the checked king whenever a rook, bishop or queen gives a line check. The piece immediately behind the king may be won after it moves; long diagonals are especially easy to miss when several empty squares separate the targets.