The voting method for Tower Hamlets' new mayor is the same as for all directly elected mayors in England:
supplementary vote. (Wellington, mentioned yesterday, uses
Single Transferable Vote.)
Voters pick a first choice candidate and a second choice candidate. If one candidate gets over half of the first-choice votes, they become mayor. Simples.
Failing that, there's a run off between the two candidates who received the most first-choice votes, everybody else gets dropped. All the second-choice votes of the excluded candidates are counted. Any secondchoice votes for the candidates left over are added to their total. The candidate with the largest combined total wins.
It's a bit odd and I'm not sure about it. Sadly, I'm no Nicholas Whyte, so that's the extent of my learned analysis. :-)