Here is a little study in failure. See Black’s pieces lined up on the long diagonal and you naturally consider Qa1; this position is similar to one we saw very early in our study of basic pins. At first it looks like the result is a relative pin of Black’s rook to his knight, as the knight is loose. But Black can both move the rook out of the way and add its protection to the knight with Rd6. That’s not a reason to give up; the knight still remains pinned, this time to its king, so the position has turned into a new challenge: since the knight is then attacked once and guarded once, consider whether you can throw another attacker at it. All you have for the purpose are pawns, so imagine marching them forward and what countermeasures Black would have available. You start with g3-g4, to which Black must reply f5xg4; now you have h3xg4, planning to play g4-g5 next and then take the knight a move later. But Black isn’t done. He responds to h3xg4 by playing h7-h6, and now he is ready to take your pawn when it steps forward to g5. Black thus saves his knight, which means White’s original idea of Qa1 doesn’t win any material after all (though it probably remains White’s best move because of its positional consequences).