This article contains a piece of theory, in this case about 'Ruffing: sometimes it's good... but sometimes it isn't', followed by some examples.
During November 2010 three exercises will follow, one in each of the next three weeks. Click 'Varia***' to read them or click the link below this article to read the first one.

We establish a suit as trump suit because we have more cards in that suit than the opponents (if the bidding has been good, that is). That way our side is in control; we can compensate for our weaknesses with trumps, for instance by ruffing a suit played by the defenders.
Still, many situations occur in which we should refrain from ruffing. There can be many different reasons for not ruffing: preserving an entry, preventing an overruff or losing trump control.
Three examples from IMP play:

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

West leads the ♠A and continues with the ♠K.
Declarer has lost one spade trick and is in danger of losing three club tricks.
Of course he can ruff in dummy, hoping the trump suit to be 2-2: after drawing trumps he can then unblock the ♦AK and cross with the ♥2 to dummy's last trump, in order to pitch three clubs on dummy's remaining diamonds: eleven tricks.
But if the trump suit is 3-1, he cannot ever enter dummy. He would have to forget about dummy's three remaining master diamonds. He wouldn't even be able to play a club to the ♣K, hoping for East to have the ♣A.

Since West cannot successfully attack clubs, there is a much better plan: to the second trick, on West's ♠K, declarer discards a club from dummy. If West plays back a trump or a diamond, declarer draws three rounds of trumps, unblocks the ♦A and ♦K and crosses with the ♥2 to the ♥8 to cash the master diamonds, pitching three clubs: ten safe tricks (yes, if the trump suit had been 2-2, declarer could have made an overtrick by ruffing the second trick after all).
And if west, after the ♠AK, were to play a third round of spades, declarer ruffs in hand with the ♥J, not the ♥2!

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

West leads the ♥Q.
Declarer counts four (possible) losers: a diamond, a club and two hearts. He does best by winning with the ♥A and return the suit. If East follows suit (if not, he cannot hurt South by ruffing, since his trump will beat air), South wins with the ♥K and plays a third round of hearts. If the suit turns out to be 3-3, there wasn't a problem from the start; South's fourth heart is a master and 4♠ made. And if West shows out (unlikely: who leads a doubleton queen towards a very strong balanced hand?), East will have to have the ♠9.
But here West wins the third heart, East showing out. If West now plays the fourth heart (if not, South will do it himself later), declarer may of course ruff with the ♠7, gambling that East doesn't have the ♠9. Much better however, is to discard dummy's ♣5 on the fourth heart. This is a loser-on-loser play: declarer interchanges two losers, the dangerous heart for the safe club. Later he ruffs the second club from hand in dummy. It is very unlikely that the second club is (over)ruffed by an opponent.

Sometimes declarer loses trump control by ruffing early in the play:

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

West leads the ♥A and continues with the ♥K.
There doesn't seem to be a problem: declarer has lost one trick and, after drawing trumps, forces out the ♦A. He can pitch his fourth club on dummy's fourth diamond. So the club suit doesn't even have to be 3-2. After all, after having lost the first trick and the ♦A, declarer loses only one more trick: a club.
If this is South's reasoning, he will ruff the second heart. He will thus be in for a nasty surprise: at the second trump trick, that suit turns out to be 4-1, and he's in trouble. If he draws all trumps, he will have none left: as soon as EW gain the lead with the ♦A, they will cash their hearts...
If declarer therefore, after two rounds of trumps, stops and turns to diamonds (which is the best he can do at that stage: dummy's third and last trump takes care of the heart suit), East will duck the first diamond trick. Declarer, who still cannot afford to draw the last two trumps, cannot but play another diamond. East wins with the ♦A and gives his partner a ruff. EW still have to make a club trick: down one.

Declarer was wrong to ruff the ♥K at the second trick. He should have discarded a club. If West then plays a third heart, dummy can ruff, enabling South to keep his trump length in tact. Later declarer pitches his other club loser on the fourth diamond, so this loser-on-loser play doesn't even cost him a trick (meaning this is the right play at match points too).
If West does not return a heart to the third trick, declarer wins, draws four rounds of trumps and sets up the diamonds: no problem.

Make yourself familiar with these techniques: try the three exercises, later this month to be published on Bridgevaria.com, in Varia ***.
For the first exercise click: Bridge declarer play: exercise 1 with 'To ruff or not to ruff'.