There was an example in the SBU Simultaneous Pairs on Tuesday evening. You are in three notrump and wish to make four club tricks from this combination:
You start with the king of clubs and both follow small. Then you lead the two of clubs towards the North hand and West follows small. Do you fly with the ace or finesse the ten?
Hopefully everyone can see that it does not matter what you do if the clubs are breaking 3-3. Another layout, East holding the remaining clubs, is irrelevant since you can never make four tricks. This leaves two layouts of interest, those where West hold four clubs and those where West holds them all.
But when West holds five clubs, we can never make four tricks in the suit. So that is (largely) irrelevant. So we should focus on the 4-2 breaks.
At this point I think many just give up. It is all mathematics, computing the probability of West holding four clubs including the two unseen honours is just too difficult at the table.
But there is no need to drag out your logarithm tables to do this - it is very simple. You are assuming East has one remaining card - is it an honour or a small card? Well, there are three cards unseen, the queen, the jack and a small card. So it is 2:1 that East holds an honour, so it must be right to play the ace. If you play small and East wins with an honour, then you will lose another trick to West's honour too.
So that is how to play this combination for four tricks without breaking a sweat.
End of lesson?
No, because there is a little more to this. Perhaps it is not essential to make four tricks in this suit. Perhaps you can afford to make three tricks and get others from elsewhere. Does this remain the best line?
Unfortunately the answer is that it is not the best line for three tricks. And here you do need to know the probabilities. But the chances of a 5-1 break are significant enough to make it right to insert the ten on the second round. This is because the combined odds of QJxx and QJ8xx outweigh the odds of Hxxx.
So there is one line for three tricks and a different one for four tricks. Which should you take for the optimum number of tricks? Actually more important is the entire hand and what tricks you need, who you'd prefer to lose the lead to, any factors that might influence the distribution. It is rare for there to be no other information that just the single suit. But, as it happens, the best line at matchpoints, in total isolation, is to play the ten of clubs on the second round.
However change the six of clubs into the eight in the North hand and you should revert back to the original line of playing the ace!
This is why suit combinations are fickle.