This is useful when the opponent's overcall has used up your bidding space and prevented you from describing your hand.
1C, (1S), X is the same as a 1H bid 6+ pts and 4+ Hs
1C, (1D), X is 1H and 1S, ie. 6+ pt and 4+ Hs and 4+ Ss
1H, (1S), X is 1C and 1D, ie. 6+ pts and 4+ Cs and 4+ Ds
1S, (2D), X is 1H and 6+ pts and 4+ Hs (not strong enough to bid 2H)
1C, (1H), X is 6+ pts and exactly 4 spades. Bid 1S with 5 spades.
In all these cases the double denies the ability to make a natural bid.
You can agree with partner to play negative doubles at a higher level.
* at the 1 level 6 points +
* at the 2 level 8/9 points +
* at the 3 level 10 points +