There are two doubletons in North’s hand, the A-9  and K-Q .  The the lower ranking card is said to guard the higher card.  In this case, the 9 guards the Ace, and the Queen guards the King.  Typically the guards are used as a waiting card in order to keep the stronger cards, but the role of the guard can be changed into an offensive weapon.

Lets look at an example game to see this tactic.

South calls up the 10 and discards the 9. West is in a dreadful position. Leading trump when defending, usually benefits the trump maker. If West leads any club, it will likely be ruffed. Therefore, West is forced to play the K. North plays the A, and wins the trick since East and South must follow suit.

Many beginners would play the A hoping the card makes it around the table, but North fires back and throws out the 9.  The once lowly guard of the Ace will change roles and go out on the attack.  North realizes that since there are only two more spades left in play, South has a strong chance to trump the 9. In fact, South has a stronger chance of winning the trick than the A making it around the table.

Play proceeds with West following suit, South played 10, and East played 10.  North and South went on to score a point.