Counting the opponents hands to determine their respective distributions gives you a big edge over the declarer who does not bother to count at all. Sometimes you can get an accurate count after eight or nine tricks have been played. Occasionally you can do so much earlier. And on some hands you never can get a count. It all depends on the make-up of the particular deal in question.
North dealer. North – South vulnerable
In today’s hand Mrs. Keen’s contract was not in danger but she gained an extra trick by careful counting. Mr. Abel led the ace of hearts and followed with the 
10. From this it appeared that he did not hold as many as four hearts.
Confirmation
With four, he would have led his fourth highest at trick two. This confirmed Mrs. Keen’s assumption that Miss Brash had five or more hearts for her overcall. Miss Brash won the second trick with the jack of hearts and shifted to the four of clubs. Mrs. Keen’s problem was to find out who had the queen of diamonds and as you see she could finesse either way against that card.
She won the third trick with dummy’s king of clubs and cashed the king of spades. Working for a count on clubs, she led a small club from dummy and ruffed it with the ten of spades. She cashed the ace of spades and noticed that each defender had started with two cards in that suit.
Last Heart
Now she led her last heart and ruffed it on the board. Next came dummy’s ace of clubs on which the trey of diamonds was discarded from the closed hand. When dummy’s last club was led and ruffed, the count was complete. Here Mr. Abel discarded a diaimond. Since Mr. Abel had started with three clubs, it was clear that Miss Brash had started with five, Miss Brash had also started with five hearts and had followed suit to two leads of spades. Therefore she had just one diamond. Mrs. Keen led the king of diamonds. She then led the nine of diamonds, and confidently let it ride; knowing Miss Brash—had no cards left in the suit.