It was suggested to me that I share some of the questions from members and my attempts at answers. This makes sense because it takes me as long to send it to one player as all. The procedure will normally be for me to answer the question to the person asking. They will reply that my answer was helpful or not and I will decide whether to send it on.
This week's question was from an online game, so I don't have the hands available. The client was East with 4 HCPs (an Ace) and two 4-card suits. The bidding went as below:
If both players were Monday attendees, they would know that a double of No Trump is always for penalty. West had already passed, but thought she could get 7 tricks in No Trump. Whether North redoubles or not, East must trust partner to still have 7 tricks and butt out. Anything he does may well make it worse. When N/S make it, West will probably say that it seemed like a good idea at the time and they'll go on to the next board.
If East and West have agreed that a double in that situation is for takeout, they will know what to do when it's redoubled. Both players know that they can't let the opponents play 1 No Trump redoubled. East must help partner by describing his hand. If he has support for all three unbid suits, he will pass. This tells partner to bid his best suit and count on East to have help. If East has at least one suit that he doesn't want partner to bid, he must bid his best suit to throw partner a lifebuoy, even with no points.
The problem, of course, arises when East doesn't know what partner's double meant. He must do the best that he can. Partner has previously passed, so it's unlikely that she could hope to set them with a flat hand, especially since you could have nothing. If she had one good suit, she would probably bid it or trap-pass. Assuming she has several decent suits and it's for takeout, she's in a jam when opener redoubles.
When East decides whether it is a penalty or takeoff double, he will follow the procedures from above. If he decides that it's penalty, he'll trust partner and pass. If he decides that it's takeout, he'll bid one of his 4-card suits since he doesn't want partner to bid the wrong suit. If he guesses wrong about the double, it will turn out poorly. That's the problem of playing without partnership agreements. There's no need to find fault, just make an agreement and get it right next time.
Stay well, Dave