North with 13 points opens his three-card club suit since he has no five-card major and diamonds are also three cards long. (One opens the longer minor, but if they are 3-3, one opens a club.) East passes and South has an opportunity to bid.
Reverses and jump shifts are strong bids when made at opener’s second chance to bid. Jump shifts by responder can be weak or strong depending on partnership agreement. I suggest making responder’s jump shift weak.
With a good number of points, a partnership bids slowly using the bidding room to investigate game or even slam. Without points, meaning the opponents have them, the partnership wants to use the opponent’s bidding space without significant penalty.
So responder’s jump takes away bidding room because he knows the opponents could use it.
It also warns partner not to get excited with bidding more or doubling for penalty.
Weak Jump shift
It is a bid of two of a major when partner opens a minor. It shows zero to six points with a reasonable six-card suit. Whether one decides to do it or not usually depends on the vulnerability and the trump-suit quality.
Does one weak jump shift in a minor over a major? No. One does not want to be in a misfit at the three level.
Does one bid a weak jump shift in spades over hearts? No, because you are not afraid of opponents finding a major.
South bids two spades, and West cannot venture a heart bid because the suit is only headed by the jack. South is left to play two spades.
The contract: Two spades by South
The opening lead: The jack of hearts
The play: Declarer ruffs the second heart and plays the jack of spades. West covers and declarer wins the ace. Declarer switches from trump and ruffs the last heart. When the opponents have only one trump left and it is a winner, declarer usually decides to leave it alone and carry out his agenda. In this case, there is no need to let West take two trumps for one of his.
Declarer runs the jack of clubs through and it holds. Declarer takes the finesse again and cashes the ace. Between declarer’s hand and the dummy, there are no more hearts and no more clubs. The hand is stripped. When West is thrown in with a diamond, he cashes a spade and his last diamond. He then has to play a further heart. This sluff-and-a-ruff allows declarer to pitch a losing diamond.
The result: Two spades making plus one for +140
Note: If South were vulnerable, his spades should be a little better. Being able to control the trump suit is a higher priority than having side-suit winners.