The bidding: The bidding in case A. is what is done before new minor forcing is adopted. North will bid four-card suits up the line, but will skip his four-card heart suit because he has five spades. When North rebids two hearts, he is showing at least ten points and five spades and four hearts. He could have five hearts, but that takes another bid to show.
The bidding in case B. is with new minor forcing. Two diamonds says nothing about any diamond length but tells partner he has 10 points and has five of his bid major or four of the other major.
South bids two hearts because he has four of them and bids the four card major before his three-card major. If partner bid two notrump, he would bid three spades showing three-card spade support. This is not necessary because a four-four fit is better.
So why is a 4-4 fit better than a 5-3 fit? It isn’t always better depending on the high card strengths of either suit. The 4-4 fit allows declarer to choose the hand to do the ruffs and the hand to draw trump. Once trump is drawn, losers can be discarded on the five-card suit. South signs off in three hearts because partner is short in the minors. Shortness opposite KJ or QJ can be a problem.
The contract: Three hearts by South
The opening lead: The six of hearts
Because of North’s shortness in the minors and South bidding clubs, West leads trump. It is never “when in doubt, lead trump.” West wants to protect his flimsy minor holdings.
The play: South finesses the opening lead and East wins his king. East leads a diamond. South ducks and West wins the Queen. Back to East’s ace, East and West cash their two clubs for down one.
The result: Three hearts down one for -50.
-Had West led a small diamond, South makes 140.
-With new minor forcing being used, case A. bidding sequence could still be used and would show five spades and four hearts with less than 10 points.