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 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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Warning: Don't try this at home unless you have a partner who is in on the secret.
Alerting Exclusion Blackwood: This falls under the category of "delayed alert." That means that you say nothing during the auction. When the auction is over (presuming your side is the declaring side), before the opening lead is made, either player informs the defenders what the bidding meant.