You have a moderate hand: S 85 H KQ53 D Q976 C Q52. Partner opens 1C and you are all set to bid, when there is an overcall of 1S on your right. You were about to bid 1D or 1H (see note below), but that is no longer possible.
You have 9 HCP and just 4 cards in each of your suits, so you cannot bid 2D or 2H. Either of these bids promises 10 decent HCP and a five card suit. These bids are also forcing - partner MUST bid again.
You lack a S stopper, so bidding 1NT is out. Even if you reverse S and C holdings, bidding 1NT is not a great option, because your reasonable H suit gets lost.
The solution is to double. This is called a negative double and asks partner to bid - it is just another form of takeout double. In this sequence of 1C (1S) X, your double ideally shows the unbid suits, with emphasis on the unbid Major, and also tolerance for partner's suit along with 6+ points.
The only time partner should pass is when the other opponent bids and he has no support for the suit(s) you have shown with the double.
Negative doubles are not restricted to weakish hands. Imagine you hold S AJ9 H KQ74 D QJ64 C J53, you still double and do not bid 2D or 2H. You double to see what partner does next. You are going to finish in at least game, holding a good opening hand opposite partner's opening, It may be 4H, 3NT or even 5 or 6C.
Imagine now that the bidding has gone 1D from partner and 2C overcall. If you hold S Q974 H K853 D 86 C K53 you can double to show both majors and 8 decent points. You will not mind which suit partner chooses (perhaps a bit nervous about 2D).
With the same bidding sequence, you now hold S J8 H AQ96 D K843 C AJ9. This is a good opening hand, so you should be in some game. Get some help from partner to find out which game - double first to show your good hand. If partner shows S or repeats D, then you will bid 3NT next. You will raise 2H to game of course.
RESPONSES TO NEGATIVE DOUBLES
As in the case of other takeout doubles, you should never pass a negative double, Partner has shown some values and some length in unbid suits, so bid. Say the bidding has gone 1C from you, 1S overcall and X from partner, so what do you do with the following hands:
(a) S 952 H A842 D 6 C AKJ64
(b) S 98 H AK65 D A8 C AKJ53
(c) S KQ5 H 8 D Q64 C AQ432
Hand (a) looks fairly minimal if you just count this as 12 points. However, these are very good points and you also have a singleton. You should probably bid 2H, just as if partner had made a 1H bid. You are close to being able to jump to 3H - in fact if the other opponent had bid 2S, I would try 3H on this hand.
Hand (b) is very strong and you should bid this straight to 4H. the 19 very good HCP, plus shortages will help make game opposite the weakest double. If you know game should be on, then bid it. If you are among the players who open this hand 2C, you deserve all you get. The hand will only take around 5 tricks if partner has nothing and you have to lead everything from hand. Open 1C, see if partner can respond and then bid strongly, If it is passed out in 1C, you were never going to make game.
Hand (c) has some decent values in their suit, so you should rebid 1NT. Partner will have values in H and probably D. You might bid 2C, but partner with an opening hand will never be able to bid NT and you have missed your chance to get to what is probably the best place to play.
These are virtually the same as negative doubles, but made by the other side. LHO opens 1H, partner 1S, RHO 2H. You hold S 86 H 6 D QJ864 C AJT53 so you can double to ask partner to bid one of the other two suits. If he repeats S, it should be with at least 6, so you will not be totally upset.
Right back to that first hand, what were you about to bid after partner opens 1C? Some believe it is best to respond 1D (bidding up the line) so that partner has the greatest possible amount of space. This is the normal way for ACOL players. Others, such as Mike Lawrence, say you should always get your Major into the picture and bid 1H. His comment is that if LHO bids 1S or 2S after you have responded 1D, partner may not be able to show a H suit. When the bidding returns to you, you are too weak to show another suit.
I would strongly recomment 1H with this hand, even if the D suit is AJ63 and H J952. They are not called Major suits for nothing.
1 do not bid at the 2 level unless you have 10 points
2 As opener,do not rebid a higher suit than your first without a strong hand. EG 1C - 1S; 2H is called a reverse, a higher ranking suit than you first bid. It shows a shapely hand of 16+ points.
3 Make a negative double after your RHO overcalls to show some values and 4 cards in unbid Major(s)
4 Do not bid NT unless you ahve a stopper in opponent's bid suit.