The really short version? It’s utterly rubbish. But this is how it works, courtesy of a short film by the Green Party:

Note: This is not an endorsement of the Green Party. But their logic for preventing BNP leader Nick Griffin from getting a seat does have its merits. I’d prefer it if you researched the parties and their candidates and cast your vote based on who tallies most with your views, but if you must protest vote, with a likely low turnout at the upcoming elections we’re in a situation where votes for the Greens and UKIP are more likely to be the clinchers that will help prevent the fascists from getting a seat than ones for the Tories, Labour or the Lib Dems. (Yes, I think UKIP are more or less a joke party with ill-thought-out policies and some unpleasant undertones to much of their rhetoric, but at least they aren’t Nazis.)