Exclusion Blackwood
I can't believe I am writing this article.
I wouldn't recommend this convention to anyone other than a full-time expert with a full-time regular partner.
Still, it is a sexy convention, and I get asked about it all the time.
This is a way to ask for aces outside of a suit in which you are void. For example, suppose you hold:
KJ107632 
AK 
-- 
KQJ2.
Your 1 opening is raised to 3 (limit raise). All you want to know is how many aces partner has outside of diamonds. Regular A 4NT bid to ask for aces (or in RKC, keycards) Blackwood (a no-no when holding a void) won't do you any good. If partner has, say, one ace, but it is in diamonds, you have no slam. If he has 2 aces, you might belong in 7 (if the aces are black), but only 6 if one of the aces in diamonds.
By jumping to 5 ("Exclusion Blackwood") you are asking for aces "excluding" the suit you jumped into (diamonds, in this case). Partner will bid the first step for 0-4, next step for 1, next step for 2, etc. So, 5=0 aces outside of diamonds (note that with only the A, this is the answer partner would give). 5=1 ace outside of diamonds and so on.
Since anyone reading/using this likely plays RKC, presumably the jump would actually ask for "Keycards outside the void suit." So, the answers would be (assuming 1430), Step 1 = 1 or 4 Keycards outside the void suit, Step 2 = 0-3, etc.
So, when is a bid Exclusion RKC? The asker must jump to a suit above the game level. That unusual bid shows a void and asks for keycards outside that suit. Typically, there will be an agreed trump suit, but if not, it would be in the last-bid suit. You must be absolutely sure the bid can't logically be natural. Here are some examples:
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