This is a form of takeout double and is usually used to show ‘I’ve got 4+ cards in the other unbid Major’.
A negative double is only used after your partner has opened and has been (non-jump) overcalled.
The maximum level at which a negative double is usually used is up to 3S. After 3S a double is for penalties.
To use a negative double the responder must have at least 6 points at the one level, 8 points at the two level and 10 points at the 3 level. There is no maximum point count (unless agreed) for a negative double. The responder can show a stronger hand in the next round of bidding.
The opener must rebid now knowing that the responder has at least 6 points and a 4+ card in a major suit.
If both majors have been bid then a negative double promises 4+ cards in a minor.
1C - 1S - DBL promises 4+ hearts and 6+ points
1S - 2H - DBL promises at least one minor and 8+ points
1C - 1H - DBL promises 4 spades and 6+ points (If you have 5 spades, just bid 1S)
A negative double can also be used when the responder has a weak hand with a long major suit and doesn't want to bid at two level but wants to show the major.
After a 1NT overcall, a double is for penalties.