Board #6 at AIB on August 9, 2018. I am repeating myself again: No trump slams are just arithmetic. North had 19 HCP; South had 14 HCP. 6NT should be a slam dunk, but only one pair bid it. South's hand was ♠7 ♥AQJ9 ♦KQJ ♣AQ764 and North's hand was ♠AK2 ♥K73 ♦1082 ♣KJ103. If East opens 3♠, South doubles. North bids 3NT and South bids 4NT (quantitative) and North goes to 6NT. That's the theory, at least. If East passes, South opens 1♣. North bids 3NT (13-14, balanced) and South bids 6NT. (You have a minimum of 32 HCP with a good 5 card suit for an extra point.)
Years ago, Rixi Markus, a superb female player, won a prize for a bridge tip of the fact that sometimes one defender has to lead LOW from honor doubleton in order to protect partner. This can occur when you can tell that a certain suit must be attacked to develop a trick for your side. If, however, you lead top of an honor doubleton, it requires that partner hold Q10x(s) opposite your Jx OR J10xx opposite your Qx in order to be able to lead the suit again from her side of the table. That is not always the case. By leading LOW from your doubleton honor, you preserve more options for the defense. You also protect partner from being finessed on the second round of the suit when she has two honors, but Declarer is sitting behind her with one or more honors.
A case in point was Board #5 at AIB. As West I held ♠Q7 ♥52 ♦J764 ♣AQ943. Against the 4♥ contract, partner (East) led a top of nothing the ♠9 which went Jack, Queen, Ace. Dummy held: ♠KJ ♥A10874 ♦Q83 ♣1072. Declarer played to the ♥A and I (West) played the ♥2—trump suit preference for clubs, the lowest suit. Declarer played a low heart to my ♥5 and (for complicated reasons) decided to play my partner for a singleton heart and finessed, losing to her Queen. Partner—having noticed my suit preference signal shifted to a club. Unfortunately, she shifted to the ♣J. Rixi would have told her to play the ♣6 instead.
I took the ♣A (because I know one of Dummy's clubs can go away on the ♠10 in Declarer's hand. I did NOT have any good options at that point. If I lead a low club, Declarer can let it ride to the 10 in Dummy, since he “knows” I have the ♣Q as partner has led the Jack. If I attack the diamond suit, I am certainly giving up a diamond trick for our side if Declarer has King and partner has Ace. (If partner has King and Declarer has Ace, diamond will not hurt.) Look at the difference if partner leads a LOW club. I can take the Ace and return a low club because now BOTH the Jack and the Queen are still out, so Declarer MUST take the King and then has a losing club with which to deal.
Bottom Line: When defending, be aware that often LOW from a doubleton honor (particularly the Jack or Queen) is the right play. Don't worry about partner miscounting the hand. Partner will know this play is a possibility and will bear that in mind (along with remembering the bidding and taking into account how many cards she and Dummy have in that suit.) The key is letting partner know that you have an honor, so it is safe for her to return the suit.
Side Note: Of course, if the ♣K had been in Dummy, then the ♣J was definitely the correct card, hoping partner has AQxx or even AQ10x(x).