Calder Valley is an interesting place, election wise. A fair margin under FPTP keeps the Tories ahead, some 6-7k votes ahead. But still, this is only 40% of the total vote.
Under FPTP there is some tactical voting that can go on. If Labour wanted to push the "two horse race" angle, and the Lib Dems followed, Labour could win in Calder Valley...not necessarily too likely though. Lib Dems locally would almost have to give up.
Under AV though there is an opportunity for a simple tactical vote, though not perhaps how you'd expect.
Due to the percentages, 39.39%, 26.97% and 25.18% for Tories, Labour and Lib Dems in that order, AV can open things up to electoral tactics.
If people were honest then you could probably expect that one of two things would happen. Either the Lib Dem vote would heavily sway to Labour, in which case Labour may well just about sneak a victory, or the vote would split and go to both Tories and Labour...in which case Tories would still win.
The Tories may well think...I'd hate for Labour to win here, I want our MP to know we prefer someone else...so 930 voters for the Tories go and back the Lib Dems, putting the Lib Dems ahead of Labour now by 1 vote. Suddenly Labour are the third party. They're not going to give their votes to the Tories, only to the Lib Dems (and who knows, maybe not even the Lib Dems any more!).
The result? Again, either the Tories win...or this time the Lib Dems might sneak a victory. Labour would be robbed of their possible slight win because the Tories manufactured the Lib Dems to come second.
But this situation is extremely rare. It requires a) for the difference between the first and second placed candidate to be less than half the total vote of the third placed candidate in the penultimate round (if we follow voter polling on how often they'd transfer), b) the Tories and Labour to be first and second, mainly because they don't tend to try and get one another to win instead of the Lib Dems (though this may change) and c) requires the change of place between second and third place to be achieved through a small percentage (a realistic number) of tactical votes...perhaps around 1000-2000 for a typical constituency.