A short reminder that when attitude and count are known or irrelevant, you should be giving your partner SUIT PREFERENCE with your carding.
You hold ♠J10653 ♥AJ3 ♦873 ♣94. LHO opens 1NT; partner passes; RHO bids 2♦ (transfer). LHO accepts transfer; RHO bids 2NT and LHO goes to 3NT. Partner leads the ♣5 and dummy hits with:
♠Q9 ♥Q10982 ♦K5 ♣J763.
Declarer plays low and you put in the ♣9 which fetches the ♣10 from Declarer. Declarer next plays the ♥5 to dummy's ♥8.
Your partner plays the ♥4 and you take the ♥J. You have to decide between switching to the ♠J (hoping your partner has something decent in that suit); continuing clubs despite that ♣Jxx remaining in Dummy (hoping partner's clubs are really good) or trying a passive "top of nothing" diamond.
You should expect two things: (1) Partner has the ♥K. If Declarer had the ♥K, he would have played it first and then tried a heart to the ♥8. (2) Declarer probably has ONLY two hearts on this auction.
So, partner's heart—since you both expect Declarer to have only two hearts—should be SUIT PREFERENCE. Partner is desperate for you to return a club. If partner wanted a spade shift, she could have played the ♥7 instead of the ♥4.
A passive diamond could easily finesse partner out of ♦Jxxx if Declarer has ♦AQ109 and the ♠J could give up a spade trick if Declarer has ♠AK8x. You should EXPECT Declarer to have four spades and four diamonds because he is known to have exactly two hearts (from auction) and presumably three clubs from partner's lead. (Since you have not seen the ♣2, partner could have five clubs and Declarer only 2 but that would put Declarer with five diamonds and four spades and he MIGHT have opened a diamond in that case. Plus, a 4-2-4-3 pattern is much more common than 4-2-5-2.)