At the vulnerability North has just enough to raise his partner’s weak jump shift to game.
Declarer wins the opening heart lead in dummy and plans to start on trumps. A finesse of the spade jack seems routine, but there are a couple of reasons to play to the king here. First, the overcaller is more likely to hold the ace. Second, looking only at the spade suit, playing to the king or jack will be fine on a 3-3 split. With spades 4-2, however, finessing the jack will work best if East has Q-x, but leading up to the king is better when East has A-x or when West has Q-x.
South therefore plays to the spade king and follows with a low spade to allow for an honor falling in either hand. Playing the jack here would establish two tricks for West, with nothing to gain.
Is that the end of the matter? Not quite. The opening lead means that East expects no heart tricks for his side, and dummy’s strong diamonds suggest a club shift will achieve nothing. The best chance is to find West with the spade queen and a doubleton in diamonds, in which case a low diamond shift will preserve the entry to the East hand.
Declarer can grab the diamond 10 and knock out the spade queen, but a diamond to the ace followed by a diamond ruff will sink him.
What if declarer has 6=2=1=4 distribution, and West has two natural trump tricks? Maybe West can helpfully follow with the spade nine and then the spade seven, thus effectively denying a second natural trump trick.