Jeff Bezos said: "Percentage margins don't matter. What matters always are dollar margins: the actual dollar amount."
Bridge players are valued on the number of tricks they take, not on the percentage probabilities of the various plays. However, most of the time percentage plays will be more profitable than antipercentage.
In today's deal, South is in four spades. How should the play go after West leads his singleton?
South had a modern-era weak two-bid. In the good old days, it was frowned upon to open a weak two with a void, because the hand was playable in three suits, not just the one advertised by the opener. Now, though, if it looks like a weak two, smells like a weak two and talks like a weak two, it is a weak two. (If something else occurs to you, tune in tomorrow.)
Assuming you employ fourth-highest leads, a two is either from a four-card suit headed by at least one honor or a singleton. Here, East knows it's a singleton. So, he wins with his diamond ace, under which South drops the six. Then East returns the diamond four, South plays his nine, and West ruffs. What next?
East is sending a suit-preference signal (his lowest diamond asking for the lower-ranking of the other two side suits), but where is the three? Did East start with exactly A-4-3 in diamonds? Very unlikely, because that would leave South with five diamonds along with his six spades. Playing the percentages, West should shift to a club. East ruffs, gives his partner another diamond ruff and receives a second club ruff for down two.