Source: http://www.vba.asn.au
One of the soundest guidelines for defence is to ‘cover an honour with an honour’. For example, in this layout:
 Q x |
||
| 9 x x x |
K x x |
|
| A J 10 x |
If the queen is led from dummy, you should cover it with the king. This will eventually develop a trick for partner’s 9. If you fail to cover, declarer can take 4 tricks in the suit. But what if dummy has two or more honours?
| Q J 9 |
||
| 10 x x |
K x x |
|
| A x x x |
If you cover dummy’s queen with your king, partner’s 10 will be finessed on the way back. But if you duck the queen, and then cover the jack with the king on the second round, partner’s 10 will score.
The correct form of the guideline is: “Cover the last of dummy’s honours.”
Let’s see it in another scenario:
| J 10 x |
||
| K 9 x |
Q x x |
|
| A x x x |
Play low on dummy’s jack. That way your side can get two tricks. But if you cover the first honour, declarer can lead back towards dummy’s 10x, and you score only one trick.
And one more thing : Like every guideline, there are exceptions:
| Q J 9 x |
||
| x x x |
K 10 |
|
| A x x x |
In this scenario, you had better cover the first honour, hoping your 10 will score later.