An interesting sequence was described to me.
Suppose you have five small diamonds, with the Ace or hearts and a doubleton clubs, or with the Ace of clubs and a doubleton diamond, nothing else.
Not very interesting.
So, LHO opens 4♠ and partner bids 4NT. You have a clear 5♦ call, so you do that. LHO comes back at you with 5♠ and partner raises to 6♦. This is passed around to Opener who persists with 6♠. Partner passes this to you. So far, this was the actual sequence.
The problem is quite involved, it seems. Partner's 4NT showed two places to play, either both minors or hearts and a minor. So far, so good.
When you picked diamonds, however, you might well have had longest hearts, medium diamonds, and short clubs. 4-3-1, 4-2-1, something like that. You picked a minor because, well, that's what you do.
When partner competed to 6♦, you know that he had both minors. Why? Because, paradoxically, it seems that he should bid 6♣ if his two suits were diamonds and hearts. If he has both minors, he just bids diamonds because he knows that you prefer diamonds to clubs. If, however, he has diamonds and hearts, he wants you back in the picture and, accordingly, must make the one call that gets you back in -- 6♣.
Now, you could protest that 6♣ could show both minors and 6♦, therefore, both red suits. But, you want to make a call in something other than trumps only when you might want to bid something other than trumps, it seems. Extra calls give extra space for informative doubles by the opponents. Plus, the preference already made for diamonds suggests shorter clubs than hearts, such that you only want to bid 6♣ on the rare hand, namely when overcaller has that unexpected length in the probable longer of Advancer's other two suits.
So, partner's pass, if you interpret it as forcing, seems to be from a minor two-suiter, making the heart-Ace-doubleton-club holding look HUGE. The club-Ace-doubleton-heart holding would have looked great after a 6♣ "heart flag."