Deal 16
 
 
 
♠ A Q 8 6 2
A 4
9 5
♣ A K 8 7
 
 WEST  NORTH  EAST  SOUTH
1♠pass1NT
pass2♣pass2NT
pass3NTpasspass
pass

North opens 1♠.
South has 11 points, enough to respond 2♣, (or 2), in standard bidding, but not when using 2/1. So she bids 1NT.

North is forced to bid again, but unlike the previous hand, here he wants to keep bidding.
He bids 2♣.
South knows North doesn't have s, so she shows her above-minimum hand by bidding 2NT.

North is happy to play Notrump. He assumes South must have something useful in s and bids 3NT.

3NT seems to be a good contract. There are 8 top tricks and possibilities for a ninth winner in either ♠s, s, or, (the best chance), ♣s.

 Deal 17 
  ♠ K
K 7 5 3
A 10 3 2
♣ J 9 5 4