This little description explains what happens:
"Like other vote swappers, A lives in a marginal constituency with little chance of her preferred candidate winning. Rather than "waste" her vote on a candidate almost assured to come third, she has decided to swap with someone in a similar boat. What both have in common is a desire to keep the Conservatives from winning in the seats where they live. In effect, A gets to vote Labour in Selby while her partner gets her preferred choice in Guildford, and both get to cast an anti-Tory vote where they live. Two votes for the price of one. The pair struck their deal on Tacticalvoter.net, a kind of virtual clearing house that pairs Liberal Democrat and Labour supporters who live in different constituencies. In simple terms, Tactical Voter - the political equivalent of internet dating - matches up anti-Tory voters in marginal constituencies where the centre-left has a decent chance of unseating the Conservatives - in other words, in the 230 seats where the Conservatives are vulnerable to a clear challenger."
Here's the link:
Keep them out
Ok, just to be fair, there are also at least 2 sites designed to give Mr Blair a bloody nose. Won't go as far as posting the links, but they were mentioned yesterday in the Online section of the paper I got my info from.