“Cover an honor with an honor” is a pernicious mantra. It should say, “Cover an honor with an honor IF and ONLY IF there is a chance to promote something in your hand or your partner's hand.” That is why covering honors in the opponents' trump suit is almost never correct. Neither you nor your partner has enough length in their trump suit to promote 10's and 9's to winning rank.
Today's example also includes instructive bidding. You pass with ♠A98 ♥Q84 ♦A109 ♣8653 and LHO passes. Your partner (in third seat) open 1♦. RHO passes. You have good spot cards but the “death distribution” of 3-3-3-4. Your 10 HCP are scattered, not really backing each other up. ♦A109 in your partner's suit is excellent, but not enough to upgrade the hand. A bid of 1NT is quite sufficient. If partner has extras, he can take action. Some people jump to 2NT with any 10-12 HCP but that is anti-percentage at matchpoints. Most of the time, partner has 12-14 points, so game opposite a 10 count is highly unlikely. Our opponent, however, jumped to 2NT and his partner passed.
Now, faithful reader, change seats: you are sitting behind Dummy, defending 2NT.
♠K654 ♥J105 ♦QJ875 ♣A
♠J7 ♥A763 ♦K432 ♣K74
Your partner has led the ♣Q and Dummy won the ♣A, as you signaled encouragement. Now, Declarer plays the ♦Q from Dummy. Do you COVER? Look at the spots. You have NOTHING to promote. You partner has either zero, one or two diamonds, so there is NO future in this suit. You must hope that Declarer has only two or three diamonds. As long as you do NOT cover, your ♦K will live to win a trick. Several pairs covered the diamond, allowing Declarer to make 2NT.
Move back to Declarer's seat.
♠K654 ♥J105 ♦QJ875 ♣A
♠A98 ♥Q84 ♦A109 ♣8653
Dummy's ♦Q wins. You continue with a low diamond to your ♦10, and LHO shows out. At this point, you are slated to go down one. You can only get three diamonds before losing the lead. The opponents will take three or four clubs tricks (depending on whether clubs are 4-4 or 5-3—the latter more likely), two heart tricks, and if you give up a diamond, they will get a diamond as well. You cannot take more than two spades without losing the lead and if you give up a spade, hoping for a 3-3 break (less likely than 4-2 once again), you'll be creating another loser in that suit. At this point, you should settle for down one and force out the two top hearts to develop one more trick.
Three Morals to this Story:
1) Do NOT bid hoping partner has the “magic hand.” 1NT response is 6-10.
2) Do NOT cover an honor with an honor UNLESS there is a chance to promote something in your hand or in partner's hand.
3) When you are doomed to fail in your contract, go down as few tricks as possible.