Support Doubles In Duplicate Bridge. They play that opener’s double shows a big balanced hand after: You can now double that one spade bid by the opponent to show that you have three hearts to support your partner’s trump suit.
The strength you need to respond depends on the level of the raise. Most of the information that follows is intended for experienced players. If opener has extra strength, he can divulge it later.
Making a support double is opener's first priority. Due to the rarity of this call, the topic is difficult and often unexplored in bridge teaching and literature. A doubleton in their suit isn't as good, but it's ok. These are the 6 articles i wrote for the acbl bulletin on doubles (in order of difficulty):