You are East and hold :         ♠ 8 3     ♥ K 9 6 3     ♦ A 7 2     ♣ J 6 5 4
The bidding went like this :

You are too weak to bid 2♥ and you can not respond with 1NT as that would now promise a stopper in Opponent's ♠ suit.   You must therefore pass.

The Negative Double solves this problem.

This shows

6-10 points & 4+cards in the unbid major suit(s)
or

11+ points & 4 cards exactly in the unbid major suit(s)

1. 1♣ - (1♥) - DBL = 6+ points & exactly 4 spades

2. 1♣ - (1♥) - 1♠ = 6+ points & 5+ spades

Opener's replies
Opener assumes that Partner holds 6-10 points and 4 cards only in the inferred major (or minor) suit(s).
With 4-card trump support Opener replies as if Responder had bid 1 in the inferred suit.
For example :

1. 1♦ - (1♠) - DBL - (Pass) - 2♥ = 13-15 points & 4-card heart support

2. 1♦ - (1♠) - DBL - (Pass) - 3♥ = 16-18 points & 4-card heart support

3. 1♦ - (1♠) - DBL - (Pass) - 4♥ = 19-21 points & 4-card heart support

Extended Usage
When the Opponents Overcall in ♦, Responder's Double shows holding both 4-card majors.
For example :   ♠ A 8 5 4   ♥ Q J 10 3   ♦ 6 2   ♣ J 9 4

Responder bids his 4-card major suit as normal when holding only one 4-card major !

When both major suits have been bid, Responder's Double shows holding strength and 4-4+ in both minor suits.
For example :   ♠ 8 5 4   ♥ 7 3   ♦ Q 10 6 2   ♣ K Q 9 4