Continuing where we left off, we could say that if declarer has 2-1, i.e. K of diamonds is the game-going trick, we will not beat it. Then our aim would be to make sure we won't fail to beat it when declarer is 1-2. If declarer's diamond is the J, partner could face a nasty decision. And if it's a low one, partner might have losing options as well. He doesn't know everything we does. Make it easy. Discard the Q of diamonds (the T would work also with the 9 visible in dummy). When declarer leads a diamond, partner will surely grab the ace and shift to something and we will know exactly how to cash out.
We could also say that we don't like to give up when declarer is 2-1. Can we trick declarer into going down, and minimize the risk of letting a beatable contract through if declarer's minor suit shape is reversed?
Our only option then is the 2 of diamonds (playing UDCA). By showing strength with the 2, partner will know we have the Q so if declarer leads a low, he'll take the ace. If declarer has Jx and believes you have the ace, he might try a 'chinese finesse' and let the J ride if not covered. Slam! The other variant is if partner gains the lead in clubs and shifts to a low diamond and declarer misguesses. Partner would never lead away from a holding including the J, so that might work. But, a suspicious declarer would probably get it right as with this particular dummy there is no hurry for the defenders to attack diamonds. So the only reason for doing that is 'trickery'.
Also, if we signal with the 2 and partner ducks a stiff J, declarer may put up the K and leave you regretting that one. On the other hand, maybe partner can deduce that you'd only try this from a 5-card suit, i.e. he'd duck the J from a Axx but win the ace from Axxx...?
Summing up: Going legit - help partner by ditching the Q. Possible con - signal with the 2.
This time either option would have worked as declarer held a glorious 9-count without the J of D.
As astout readers can gather from previous posts, the contract made at the table when the 5 was discarded and partner ducked the diamond.
Anyone wanting to add to this analysis in the 'search for truth' are welcome to post comments. We often learn the most from disasters.