QUESTION: When is it correct to double "for protection" at matchpoints? Also, are there any cases where you do it at IMPs? (Probably VERY rare because you're screwed if you're getting -470 or worse.)
I'm probably not the best person to answer these questions, because I much prefer matchpoints and rarely play IMPs.
You are correct that the conventional IMP strategy is to compete fiercely at the two level, and be willing to give up at the three level.
However, MOST pairs are reluctant to double at IMPs, so lots of people “get away with murder” in club games by bidding on, especially when non-vulnerable.
Close matchpoint doubles are a very difficult issue. Most people do not double enough. The rule of thumb is to expect at least one in ten of your penalty doubles will end up in the opponents making their bid. Otherwise, conventional wisdom is, you are not doubling enough. However, just because you think you are making your two of a major does not mean you HAVE to double their three of a minor. The major issue is: "Do you think other pairs will be bidding three of the minor with the opposition's card? If the answer is "Yes," you do NOT have to double. You are probably with the field. If the answer is "No," then you may have to double because it won't much matter whether they make 110 or 470 when everyone else lets you and your partner play at two of major. (Incidentally, in any decent matchpoint field, no opponent will let you play two of a major with a fit when you want to stop there. One of my regular partners claims he has only once seen me fail to balance on a 1M P 2M P P auction. I believe that is a slight exaggeration but generally true.
Some close matchpoint doubles are made to protect your partner—part of any good partner's duties. If they balance in and you have secondary honors in their suit that are useful for defense but probably useless on offense, you must double to protect your partner. An easy example would be that it has gone 1♠ P 2♠ P P and the next person bids 3♣. If you have a club holding that looking something like ♣Q10xx or ♣K10xx, it is probably advisable to double to STOP your partner from taking the push to 3♠ with her possible singleton in their minor. On the other hand, if you have xxxx in their suit, you have nothing wasted and will not mind at all if partner takes the push to three level with a singleton.
Another issue is how good is your opening hand? Partner will expect two quick tricks for an opening bid, but Responder may supply only one trick with a single raise. (Two tricks will happen often, but not guaranteed.) That means, opener should have a reasonable expectation of three tricks in his/her hand to make the penalty double.
If you and your partner play constructive raises of majors, you can expect slightly more from partner. In that case, I'm comfortable expecting two tricks from partner and doubling with my two quicks and a trump trick.
Responder should also be willing to double to protect partner, e.g., 1♠ P 2♠ P; P Dbl P 3♣—if responder has trump tricks, double to protect partner. If you 8-10 decent HCP and at least one trump trick, you'll probably set the contract and you do NOT want partner to take the push.