Back-to-Back consists of two puzzles (A and B) on the same theme. Yesterday's puzzle 5A and today's puzzle 5B are about:

Getting rid of a loser in the side suit

S/—		♠	6 5 4
		♥	A 6 3
		♦	A 7 6
		♣	8 6 5 4
		♠	A K 3 2
		♥	K Q J 10 9
		♦	4 3
		♣	K Q

West leads the ♦K. How should South play?

Solution

Declarer counts four losers in the South hand (the hand with the longer trump suit): a diamond, the ♣A and two spades. If the spade suit is 3-3, there are no problems. But what if it's 4-2 (a more common split)?
Many declarers oversee the possibility of a ruff in a suit that is distributed 4-3 fit. Probably because, hoping for a 3-3 distribution, they focus too much on setting up the fourth card in that suit. Often, however, it's simple to arrange for a ruff of that fourth card. And so it is here.

S/—		♠	6 5 4
		♥	A 6 3
		♦	A 7 6
		♣	8 6 5 4
♠	9 7			♠	Q J 10 8
♥	8 7 5			♥	4 2
♦	K Q J 10			♦	9 8 5 2
♣	A 9 7 2			♣	J 10 3
		♠	A K 3 2
		♥	K Q J 10 9
		♦	4 3
		♣	K Q

South wins with the ♦A and plays a club. If the defence wins and presses on with two more rounds of diamonds (best defence), South ruffs and cashes his club trick and the ♠AK (if EW duck the club, South also continues by cashing the ♠AK).
Next he plays a third round of spades. As soon as he regains the lead, he ruffs his fourth spade with dummy's ♥A. Completely safe and there's nothing EW can do about it. (Of course, if EW play back trumps twice, after winning the ♣A and the third spade, South should win in hand, preserving dummy's ♥A).

PS: Why declarer must play clubs first? If he doesn't, one of the opponents may shed clubs at one of the diamond tricks and especially at the third and fourth round of spades, enabling him to ruff South's club trick later (after all, declarer is rather likely to lose trump control, which in itself is not a problem, provided he safeguards his club trick first).
To look at it another way: the recommended line of play is in fact a cross-ruff. And in a cross-ruff declarer should cash his side-suit winners first.