Another pretty simple case. White’s king has ventured far enough onto the board to have queens to its north and south, so you are thinking actively about the skewer that would become possible if you could get White’s king and queen lined up with one another. How can you force your opponent to so align them when that is the last thing he wants to do? With checks that force his replies. Look for every check Black can give and you see five; most of them would force the king to move, but Qe1+ is the one you jump to because it’s safe and puts the king under the greatest pressure, sealing off all its flight squares on the second rank. The d-file also is out, so White’s king is driven to f3 or f4—and into line with his queen. Now Black plays Qf1+, and the skewer picks up White’s queen once the king is forced out of the way.