A true expert will have various resources at their command.
Superb declarer play lets them succeed on hands where others might fail, taking advantage of poor defense or advanced plays (say a criss-cross squeeze.)
Listening skills are useful. Is partner marked with points, even though they were unable to bid? In that event, one may be bidding on partner's unstated points. You have a weak hand, but your opponents have stopped in a low partscore. In this case partner probably has some values, but had no convenient bid. Note that playing against very weak opponents, this is dangerous, as they may have stopped in a part score despite a mountain of points.
A light opener has preemptive value. In third seat, with two passes before you, if you have a weak hand, then so does the 4th seat bidder. Again, this is judgment based. A light opener can confuse your opponents, convincing them to stop below game, or get to the wrong game, or misleading them about the location of high cards. It can do the same things to partner too, but when partner is a passed hand, the cost of that is much lower.
Understanding the true trick-taking value of a distributional hand is important, and something difficult to teach.
Minor honors and spot cards, when combined with higher honors often amount to extra tricks by a declarer who can use them.
Understand positional play. Thus, if one opponent has opened the bidding with a strong 1NT, and the other shown a weak hand, then you know where the vast majority of the points are. If your points lie above or below their values, then your kings and queens have more or less value, depending on this location. Knowing where the points are makes that opponent easier to endplay.
Know your opponents and their system. For example, opponents (especially weak ones) who use negative doubles are often unable to find a penalty double of a pushy bid.
The bidding of a hand is inextricably entwined with how you will play it, or it should be. If you can visualize how a hand will play out, then you can also know when to bid aggressively or when to stay silent.
As an example, here is a hand I had a few years ago. This hand is really about bidding, about visualizing partner’s hand and how it fits and how it will play with your hand. I’ll ask you to accept our bidding agreements on this hand, because they will help you to visualize partner’s hand. I am NOT trying to convince you to play our system, but only to learn to visualize how a pair of hands will fit based on what you learn from the bidding.
Not vulnerable versus vulnerable in a club game at matchpoint scoring, I am sitting South. East deals on my right, and passes.
South
AKT4
A82
J842
84
I have a nice 12 point hand. Since we play that 1NT shows 11-13 high card points in a balanced hand, I make that bid. Personally, I love the weak 1NT. It has great preemptive value.
Partner bids 2NT. For us, this shows a hand with focus on the minor suits. I am asked to bid my better minor at this point, so I trot out 3♦. I see 3NT from partner. Our agreement here is that partner has a hand with 5-5 in the minors or longer, with game forcing values, but not slam interest opposite the weak 1NT.
Stop now and visualize partner’s hand. I expect his basic shape to be something like
xx
x
xxxxx
xxxxx
or
x
xx
xxxxx
xxxxx
See that we will have no losers in the major suits, and that the hand will play nicely on a crossruff. My top honors in the majors should be the perfect cards to play 5♦. As well, 3NT may not have good play chances, since they will probably lead our weak suit - hearts. I choose to bid 5♦ here, and all pass quickly.
West leads the spade jack, and dummy is exactly as I had visualized. Partner has shown his hand perfectly, and although I feel his willingness to force to game opposite an 11-13 point 1NT is aggressive, I hope to justify his trust in my play.
North
Q5
6
QT763
AKJ75
South
AKT4
A82
J842
84
It appears that 5♦ has good play, while 3NT will probably fail on a heart lead. But can I make 5♦? My major fear is a spade ruff. If the jack is a singleton, I will need to draw trumps as quickly as possible. I win the spade queen in dummy, noting that my spade 10 spot is a winner. (This is a good thing to do, to make mental notes as your cards become winners. Make it a habit, and then you will not need to wonder at the end of a hand if a certain card is high. You will KNOW it is so.)
I lead a low diamond towards my hand, East playing low. Insert the jack, which loses to the ace. By the way, there was a reason I played it this way. If East has the Ax or Kx or Kxx or Axx in diamonds, I want him to play low on the first trick. I don’t want him to win the first trick and play another spade. Had I led an honor from dummy, East might choose to go up. By playing a low card, East is more likely to play second hand low.
In with the diamond ace, West exits with a diamond, to the queen and king on my right. East exits with a heart, won with the ace. I have lost two trump tricks, but now am in full control of the hand. Here are the hands after four tricks have been played.
North
5
---
T76
AKJ75
South
AKT
82
84
84
The lead is in the South hand now, so I cash the top three spades, pitching two clubs from dummy along the way. Since trumps are now drawn, the remainder of the trumps are used in cross-ruffing my losers away. Our score of +400 is a tie for top on the hand, several pairs having gone down in 3NT, and some others having stopped in a part score. After all, the two hands are only 12 high card points opposite 12. Points, smoints. Visualize the hands. The complete hands were
North
Q5
6
QT763
AKJ75
West East
J2 98763
QT973 KJ54
A5 K9
Q632 T9
South
AKT4
A82
J842
84
Indeed, West should lead a heart against 3NT, and that contract should go down, while 5♦ is the only game that makes.
Stay on your toes in the bidding and in the play.