You're today's East and hear South open one heart and West jump to three clubs, preemptive. North bids three hearts.
Your partner has good playing strength but little defense, and your defense is nil. Almost surely, North-South can make six hearts, but your side would have a profitable sacrifice at seven clubs. You face a tactical problem. Expert approaches would vary. Some would keep silent, saving only if North-South reached slam. Others would raise gently to four clubs. A few would indulge in some outrageous bluff bid.
A possible tactic is to bid six or seven clubs directly. I would adopt that approach if I thought the opponents might be cold for a grand slam and if I knew West's preemptive style was classical. I would also be inclined to take a "premature save" if my opponents were experts and could be trusted to bid accurately - and then to play accurately. A sacrifice will be a big loser if declarer is going to misplay the slam.
Suppose West leads the king of clubs against six hearts, and South ruffs with the ace of trumps. If he draws trumps, he has only 11 tricks, and when diamonds break 4-1, he loses two diamonds.
East should sacrifice if South is a capable declarer who can recognize and execute a dummy reversal. South leads a spade to dummy at Trick Two, ruffs a club high, leads a trump to dummy and ruffs a club. He draws trumps and claims with four trumps in dummy, three club ruffs, three spades and two diamonds. He would have a chance for 13 tricks.