There is a popular Bridge adage Second Hand Low. However, rules are defined by their exceptions. And the exception to Second Hand Low is Second Hand High. Even though the situations where the Defense will use Second Hand High are not that frequent, they do occur often enough to warrant consideration. One of the more common and dramatic use of Second Hand High is. . . .
preserving the entry into partner's hand.

In HAND 1 right West opens the bidding with a Weak Two in Hearts. North/South eventually get to 3 No Trump. Opening lead Q
H. Declarer wins the opening lead with the K H And crosses to dummy with a Spade, then leads the Q C. Should East play low? Note: the old saw Cover an Honor with an Honor does not apply since nothing can be promoted. If Declarer has the A C then there are 5 Clubs, 2 Hearts and 3 Spade tricks, making 3 No Trump. So East must assume that her partner has the A C. If East allows West to win this trick, West can lead a Heart knocking out South's Ace. But West has no entry back to her hand. If another Club is led and East wins, East will have no more Hearts and be unable to get to partner's hand. East must rise to the occasion and go up with the K C, preserving the A C in West's hand. East wins the trick with the K C and leads a her last Heart. Now when Declarer plays Clubs, West wins with the A C and runs the Hearts setting the contract.

Another occasion to play Second Hand High, and perhaps a bit more difficult to spot is to. . .
kill dummy's long suit.
Yes, Bridge is war and there are casualties. The loser will often get a negative score. In the HAND 2 right, South holds up for two rounds and is forced to win the third round with the A
H. Declarer has 8 tricks off the top: 1 Spade, 1 Heart, 4 Damonds and 2 Clubs. The ninth trick must come from Clubs by finessing twice. 75% chance that the Club honors (Queen and Jack) are split. South tables the 3 C. If you play low (Second Hand Low) Declarer puts is Dummy's 9 C.  East wins this trick with the J C, but due to Declarer's hold up play has no Spades to return. East plays a low Heart. South does not finesse but hops up with the A H and plays the 3 C. If you play low again, Declarer finesses with the 9 C and has the rest of the Club suit making an overtrick.


                     NORTH                                            S A K Q                                          H 7 5 4
                      D  7 5
                     C  Q J 10 9 3

    WEST                       EAST
 S  5 4                          S  J 9 7 3
 H  Q J 10 8 3 2           H  9 6
 D  10 3 2                     D J 9 8 4
 C  A 5                         C  K 7 6             
                    S    10 8 6 2           
                     H  A K
                     D  A K Q 6
                     C  8 4 2

                      S 7 2                 
                      H 7 4 3
                      D  6 5 4
                      C  A K 10 9 4

WEST                          EAST
    S K Q J 9 4            S 10 6 3      
    H K 2                     H 10 8 6 5
    D 10 8 3 2              D  9 7
    C Q 7                     C  J 8 6 2
                     S A 8 5                 
                     H A Q J 9
                     D  A K Q J
                     C  5 3