Lesson
White to move. Main line: 1. Be6+ Kf8 2. Bxa2
With 1.Be6+ the bishop checks along the a2–g8 diagonal. The king must step aside, and 2.Bxa2 collects the queen sitting behind it — a textbook skewer, the reverse of a pin.
A skewer attacks the more valuable piece first and wins the one behind it. Look for an enemy king and queen (or rook) on the same line and a bishop or rook that can deliver check through them.