Today's deal is Board 2 from December 6, 2021.  It is interesting from a couple of perspectives:

1) After South opens 1NT, should East-West compete or go silently into defense mode?

2) Are you and your partner on the same wavelength when it comes to defense?

In answer to the first question, always consider the vulnerability.  Today, N/S are vulnerable.  Should that make a difference to your decision?  Yes.  If N/S go down 1 trick in 1NT vulnerable, E/W will get +100.  In other words, if E/W were declaring in 1NT they would only get +90.  So defending 1NT will earn you a better score than declaring in 1NT.  So today it is perfectly reasonable to let N/S play in 1NT.

Conversely, if N/S were non-vulnerable, down 1 would net E/W +50.  But if E/W can make something (1NT, 2♥ , etc) then +50 would not be a good score.  So when an opponent opens 1NT and she is non-vulnerable and the opponents want to play there, if you allow that to happen your result is usually starting at average and going down.  So when they are not-vulnerable, try to get in to the auction if you have shape to your hand.

Now let us consider #2.  Today many West's led the ♦ 2.  Perfectly natural.  East plays the ♦J, South wins the ♦K and leads back the ♦9. Covering the ♦10 is good or ducking is good.  What is mandatory is breaking the communication with dummy's diamonds by NOT winning the ♦A at trick two.  If you did that, well done.

Now look at the East hand.  West has led diamonds (ever notice how often partner's notrump lead always seems to be your singleton ).  When South leads a second diamond in the hope of driving out the ♦ A, East must discard.  On defense, your first discard it critical.  West will be watching this card for a clue as to how you want her to proceed.

If you are playing standard discards, you must choose to discard a high card to encourage a suit or a low card to discourage a suit.  Today, which card would you choose?  A high heart such as the ♥7 will ask partner to lead hearts when she gets in.  The ♣3 would say you have no interest in clubs but would not say what you do want her to lead.

But the first thing when defending against notrump is to do the math.  If you are East, what do you know?  You have 8 HCP, there are 2 HCP in dummy, and South has 15-17 HCP.  Therefore West has 13-15 HCP.   West should do the same math.  When West sees the dummy, she knows that East will have 7-9 HCP.

Many East's discarded the ♣3 first.  There is nothing wrong with that. And most West's covered the second diamond from South and she won  the ♦Q in dummy and led a club.  All declarer's led a club from dummy at trick 3 and West won the ♣Q.  What should West know at that point?

Declarer likes clubs, therefore East does NOT like clubs, i.e., East has zero points in clubs and only 1 HCP in diamonds.  Hence, East's HCPs must be in the major suits.  So West should switch to a major at trick four.  Which major?  That is a guess.  But all will lead to defeating the contract today.

In today's deal, 10 out of 27 declarer's made 1NT.  Considering that this contract can go down 2, for +200 for E/W, letting NS make notrump make gets you a 19% board.

Summary:

i)  Always do the math when defending notrump.  Know the range of HCPs that your partner will hold

ii)  Make your first discard count.  Today, if you are East and want to signal for a heart, discard the ♥7 as your first discard.  Don't be cheap with your discards but also do not cost your side a trick with a discard.

iii)  Watch how declarer plays a hand.  The way they attack a hand tells you where their HCPs are and by elimination, suggests where your partner's HCPs are.