Sometimes it is hard to distinguish between good technique and what is commonly called table presence, or a talent for guessing. There is nothing mysterious or supernatural about it -- it involves mostly the use of some basic notions of probability, and common sense.
You are playing 4 Hearts, after the lead of the Queen of hearts.
Should declarer presume that the lead was from QJ10?
Of course! It is purely common sense, coupled with the fact that you must find a good break in hearts to have any chance of making the contract. (Otherwise you would lose at least 2 Aces and 2 hearts).
So, your first plan is to draw two trumps with the Ace and King; then run all the spades, ruffing the 4th round, expecting to find them 4-3, and pitching one diamond from dummy; finally, you will play a heart, endplaying West, which must give you your 10th trick upon his return (you will make 3 spades, 4 hearts, 2 clubs, and the extra trick West will give you).
Quite simple and effective. However, after playing the two top hearts and running 3 spades, East pitches a diamond on the 3rd round. West had 5 spades originally. What now?
You cannot insist on your original plan, since West will have an exit in the 5th spade. Since you will have to breach clubs at some point, you do it now, playing a club to the King. East should duck that, but that is often not so easy, and he won it to push a diamond through. You try the Jack, losing to West's Queen, who cashes his trump trick and plays the 4th spade trick. You ruff in dummy and this is the position you face:
In actual play, declarer now played a club to his 8!
My reasoning (for I was declarer) was that West had already showed up with 8 major suit cards, while his partner had only 4. Any specific card missing was much more likely to be with East, which had a 2-2-?-? hand originally. The most likely distribution for these unknown cards would be 5-4, in which case the club finesse was odd-on.
Good guessing, or technique? I prefer to think of it as a little bit of both, with emphasis on the second.