... that you get to give a first and second choice in the Mayor election, as well as a vote for a constituency London Assembly candidate and the London-wide London Assembly list.
Just in case readers aren't aware of how the Mayoral preference system works, your two mayoral preference votes work like this:
If one candidate gets over 50% of first choice votes, then they win, but if that doesn't happen then voter's second choices come into play.
If your first choice candidate isn't one of the top two after the count of first preferences, but your second choice is, then your second choice vote is added - at full value - to your second choice candidate's total.
This allows you effectively to both vote for someone and against someone else. For example: if you want to back candidate C, but don't think that they will beat candidates A and B on first choice votes, and you would rather that candidate A didn't win overall, then you could give your first choice vote to candidate C, and then your second choice to candidate B.
More details of the mechanics (but not the effect) of the Mayor vote are given here:
http://www.londonelects.org.uk/voting/v ... mayor.aspx