CP5 - 3rd player
sitting over Dummy's Honour card

must try to capture it
♥ A Q 6 2
Lead (W)
  3rd player (E)
a. ♥ K  9  4
b. K  7  4
c. K  9    

Every Honour card has the dual function of trying to win a trick as well as trying to capture one of Opponent's Honours.

In above example West is leading through Dummy's strength.
If Dummy plays his ♥Q East will capture it with the King and win the trick. (East must also cover if the ♥A is not in Dummy but in Declarer's hand.)

If Dummy plays low East should
a. play the ♥J or ♥10 or ♥9 if he holds one of them
b. play the ♥K holding only cards lower than the 9
c. play the ♥K when he holds a doubleton in the suit

♥ Q 6 2
Lead (W)
a. J
  b. 10
c. 9
  3rd player (E)
♥ K 7 4

A similar situation arises when West leads a Jack, 10 or 9.
Now East must play low when Dummy plays low.
East can not prevent Declarer winning the trick, but he must keep his ♥K to capture Dummy's ♥Q.

CP6 - The "Duck"
when the side who leads deliberately plays low in both hands
to maintain an entry or to develop winners in a suit
♦ 5   > 3
2   4 7 8 K A ♦

When Declarer holds two cards in Dummy's 6-card suit, Opponents together hold five. If these break favourably (3-2) they will always make one of the first 3 tricks.
If Dummy's ♦A and ♦K win the first two trick, Opponents will win the third trick, leaving Dummy's remaining diamond winners stranded when there is no side suit entry to Dummy's hand.
Declarer must therefore duck the first diamond trick and let the Opponents win. Now entry to Dummy's five winners is secured.

The same principle applies when Declarer holds 3 cards in Dummy's 5-card suit.   Duck the first round, then (provided Opponents' cards are split 3-2) cash Dummy's four winners.

♦ 8 5   > 3
2   4 7 K A ♦

Testing Opponents' 3-3 split
♦ 8 5   > 3
4   7 K A ♦

When Declarer's side holds seven cards in a suit (divided 4-3 or 5-2) they can win one or two extra tricks provided the Opponents' cards in the suit are divided 3-3.
Declarer's side must duck the first round. After regaining the lead Dummy's ♦A and ♦K win the next two rounds and, provided both Opponents follow suit each time, the extra trick(s) can be cashed in.

If Dummy's ♦A and ♦K are played on the first two tricks (leading a low card on the 3rd), the Defence will win the next two tricks whenever their diamonds are split 4-2.