All I remember clearly is a lot of screaming. I was, allegedly, playing with a fork in the general vicinity of a power outlet. My mother says I got close enough to take ten years off her life. The end result of the yelling was that I became terrified of both electrical outlets and, oddly, forks. Luckily, spoons work fine with most things a 4-year-old would eat anyway.
The bridge version of a fork in the outlet is giving the opponents a ruff and a sluff as a defender. While giving declarer a ruff and a sluff is potentially deadly for the defense, you shouldn’t be afraid of the constituent parts.
What is a ruff and a sluff and why is it bad? It refers to a situation where both declarer and dummy are void in a suit (and both have trumps). Playing that void suit allows declarer to ruff in one of the hands and discard (or sluff) a loser in the other hand. We’ll see why this is so bad if we look at an example. We'll see all four hands. The contract is 4 and this is the current position:
Declarer has two surefire losers: a diamond loser and a club loser. However, if the defenders lead a heart, declarer can ruff in dummy and sluff away one of the losers in hand. In that case, declarer loses only one trick. Offering a ruff and a sluff is often a good way to make friends with declarer.
You can avoid giving up a ruff and sluff if you don’t play suits dummy is void in WHEN dummy has trumps remaining AND declarer may also be void.
While you should be careful not to allow a ruff and a sluff, don't be afraid to allow a ruff if that is your safest option.
Let's test this on a full deal.
You are East, NS vul. You hold:
Partner is an aggressive preempter at this vulnerability and the auction is as follows:
You might be tempted to compete to 5 at this vulnerability, but you have enough defensive value that you might just be beating 4. Why turn a plus into a minus (and if you are not beating this, you could be going -800 in 5 doubled).
Partner leads the Q.
You win the A and declarer plays the seven. Now what?
I've banged the drum before about exiting passively. You don't want to play new suits unless you absolutely have to. The most common reasons you'd have to switch are if continuing could give up a ruff and sluff or if dummy has a source of tricks and you want to get your tricks quickly.
In this case, is there anything in dummy that scares you? Declarer is going to get to ruff a heart no matter what you do, so playing hearts is the safest choice. You "know" the position from the auction and lead: declarer started with either Kx or Kxx. Partner might bid 3 at these colors with either a 6-card suit or with a 7-card suit.
Continuing a heart will not give up a ruff and a sluff. Yes, declarer can ruff in dummy or play the K and discard something in clubs or diamonds from dummy, but that discard does not seem particularly useful. Dummy will still have lots of clubs or diamonds which aren't going anywhere.
Let's say declarer wins the king (partner playing the 9), and plays the spade 10. Partner plays discards an encouraging heart and declarer plays the 8 from dummy. You win your K. What next?
This time, hearts are not so safe! You may be giving up a ruff and a sluff if you play hearts. Instead, you need to exit safely, and the ONLY safe choice is to play another trump. Your side is never going to take another trump trick. Eventually declarer will lose two tricks in the minors to go with two major tricks for down one. Good thing you didn't bid 5!
Look at the full deal:
The clubs and diamonds both present temptations for an unwary defender, but look what declarer can do if you play either at trick two. The Q will allow declarer to win the ace and eventually (after dealing with trumps) lead a diamond to the K and take a ruffing finesse against your J.
A club switch would allow declarer to play low at trick two and later finesse for your king.
The key to being a good defender is to figure out how to avoid helping declarer. A ruff and sluff is declarer's best friend, however a more common mistake by the defenders is to start too many new suits.