A friend of mine gave me a problem from the club game.
His hand was ♠AKQx ♥AJ10 ♦Kxx ♣Qxx. After his partner opened 1♦, RHO overcalled 3♥, red against white. The practical bid of 4NT quantitative was his choice, with which I agreed. His partner's next call was 6♣, which is a slightly weird bid. Whatever it means, the principle as to how to proceed, if at all, could also fit in had the auction been different (opponents actually favorable, with a 5♥ raise, for example). If the heart holding is insufficient for what I'm about to suggest, imagine AQ or AK.
In any event, 6♣ came as a shock. My friend, not sure what to do, just blasted 7NT (matchpoints), figuring that partner had to have the wood for that call.
I suggested a more calculated approach. If you are willing to bid 7NT on a guess, then 6NT can't be out of the question. So, why not use flags here?
6♥ would be a club flag; 6♠ a diamond flag. By a "flag," this asks partner about the quality of his holding, contextually, in the flagged suit. If Opener hears 6♥ and has that extra whatever in clubs, he accepts by bidding 7♣. If he hears 6♠ and has the extra something in diamonds, he also accepts by bidding 7♣ -- no reason to assume that the asking bid is about the eventual trump suit prematurely.
Now, you can actually improve on this in two ways. First, you could have Opener reciprocate with one of the highest Last Train calls imaginable. If he hears 6♥ as a club flag and lacks that "something extra" in clubs, but does have something extra in diamonds, he bids 6♠ LTTC to announce that. Responder might be wanting to play the grand in whichever suit has the best trump holding, to guard against the obvious chance of a bad split, using spades for the obvious spade-other-minor squeeze.
With the actual hand, this is a great solution for the problem. Responder assumes that Opener has something like AQxxx in diamonds and AKxxx in clubs for the 6♣ call. He wants to know if partner has the Jack also in one of these suits. So, he bids 6♥, an "asking flag" in clubs. If Opener held AKJxx in clubs, he'd accept, and Responder bids 7NT, planning to run a spade-club squeeze for the 13th trick. If Responder has only AKxxx in clubs but AQJxx in diamonds, he bids 6♠, and Responder again bids 7NT, planning to run a spade-diamond squeeze for the 13th trick. If Responder bids 6NT, meaning no jacks, Responder can only count 10 tricks if both minors split poorly. However, once he sets one up, he runs the spade-minor squeeze in the other suit for the 12th trick.
Sure, I know. A 10 might also do the trick. Opener might also take a position with a 10.