Experts prefer not to rely on a guess if possible. Take this deal, played in four hearts on a club lead to the ace and a diamond return. Ten tricks will be easy if either red suit splits, but if hearts are 3-1 and diamonds 4-1, it might seem that declarer cannot take two ruffs in dummy and should therefore seek an extra spade trick instead. If he does, he must choose between a straight finesse and a ruffing finesse through East.
However, if anyone has a singleton diamond, it is surely East, given the opening lead and shift to the jack (with the queen outstanding). In that case, declarer can afford to play on diamonds before drawing trumps — as long as he leads through the void. This way, declarer might be able to score both a club ruff and a diamond ruff in dummy, making game without the need for a spade guess.
After winning with the diamond ace, declarer plays the heart seven, which East correctly ducks. A second round of trumps would see East win and play a third trump, so declarer should instead ruff a club to dummy and play the second diamond from the table. If East trumps in, he is ruffing air. He therefore discards. Declarer scores his king and then gives up a diamond to West, who has no trump to play. Declarer can win the spade shift with the ace, ruff a spade and ruff a diamond high in dummy. Whether East overruffs or not, declarer eventually crosses back to hand in trumps and scores his long diamond as his 10th trick.