On this hand 6S by West is difficult
North is likely to lead a heart. Declarer wins, cashes the SA, plays low to the DQ and takes the losing spade finesse.
Assume North continues with another heart, which Declarer wins. Declarer takes out the last trump with the SK.
There is no problem if diamonds were 3-2. But South's play of the D10 at trick 3 is disturbing.
Nothing is lost by leading the D9.
If North has the DJ, and covers the D9, then Declarer wins and the situation is clear. Return to hand with the CK and the lead of the D7 establishes the 4th diamond.
But what if North does not cover? The odds are 2-1 in favour of North having the DJ, and hence of diamonds being 4-1.
I know some players will disagree with this.
Clearly, if South had played a small diamond, a 3-2 split is far more likely than North having 4. But the play of the D10 changes the odds completely. Declarer should run the D9.
But of course, that will fail one third of the times, when playing diamonds from the top wins!!
My initial analysis above is wrong and in fact there is a slight advantage in not running the D9. I had ignored the possibility of a South holding of DJ108.
With this holding South should also drop the D10 or DJ on the first round, giving Declarer a possible losing option. Hence the chances that North holds the DJ are not 2-1, but roughly evens.
Further, there is the possibility that North might have carelessly covered the D9, if he had the DJ. Hence running the D9 is against the odds.