The importance of second preferences to the London Mayoral Election

Andrew Gilligan explains it rather well (my emphasis):

"How it works is this: first, they count all the first-preference votes. Every candidate bar the top two (almost certainly Boris and Ken) is eliminated. Then they look at the second preferences of those who voted (for) the... eliminated candidates. Any for Ken are added on to Ken's first-round total; any for Boris are added on to his first-round total. Second preferences for any other candidate are ignored.

That means two things, neither widely understood. To begin with, there is no point at all in giving anyone except Boris or Ken your second preference. Second preferences for Sian, Brian or the others (favoured by 67 per cent of Londoners for their second pref in today's poll) are a waste of time.

And Point B is this: if you vote for Brian, or Sian, or one of the other lesser parties as your first choice, and Boris or Ken as your second, it is your second choice vote that will count, not your first.

Across London, there does seem to be a substantial desire to kick the Mayor. But Livingstone activists' road to victory may be to claim that you can somehow satisfy that desire, and "register a protest", by giving someone else your first preference, and Ken only your second.

In practice, a second preference vote for Ken or Boris is worth precisely the same to them as a first preference vote. The only way to register a protest against either man is not to vote for them, first or second. (There is, incidentally, no obligation to cast a second-preference vote.)

...

In a tight race, second preferences could be really important. So it's vital that people don't cast them without fully understanding what they're doing."

Spread the word, so people know what the voting system means. If I had my way then only people who understood the voting system would be allowed to vote. Mind you if I had my way then only people who had paid income tax in the last 3 years would be allowed to vote...