Overcalls : Deal 3
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Dealer is W     None Vulnerable
North
♠ Q 6
♥ A Q 10 5 2
♦ K 9 6
♣ J 10 4
Next
South
♠ K 10 4
♥ K 9 8 3
♦ Q 10 4 2
♣ 5 3
 
W N E S
1♦ 1♥ X# 2♥
2♠ Pass - 3♥ *
- Pass** -  
 
# = Negative Double, showing to hold 4 spades exactly.

*= With 4-card trump support South raises competitively only.
      (Following the "Rule of Total Tricks")

** = North must not raise Partner's competitive bid to Game.

Rule of Total Tricks
When the HCP between the two sides are not unequaly divided, each side will make as many tricks as their combined trump holding (but subject to the favourable or unfavourable position of their Kings and Aces).

In above hand South (holding one more trump than promised) knows their combined trump holding is 9. He can therefore competitively raise their ♥s to the 3-level.