Daniel Finkelstein has just posted something on exactly the lines I was about write:

Clare Short's decision to campaign for a hung parliament...has left me confused. How exactly does she want voters to behave?

Obviously, you can't vote for a hung parliament, you have to vote for one of the party candidates in your constituency. But how do you know which one to vote for in order to help Ms Short's campaign? You would have to be able to guess how the election was going to end up (nationally and in your constituency) in order to make sure you cast your ballot correctly.

When I heard her on the radio this morning, her argument was unintelligible. It is literally impossible to vote for a hung parliament. One can vote for a party or against a party. But unless Ms Short intends to draw up for herself the dispensation of seats across the country which would lead to a hung parliament, and then issue specific instructions every voter in the country so that their votes would lead to such an outcome, she is talking complete nonsense. And since she cannot do what I have just suggested she needs to do, she is...talking complete nonsense.

All one can do is guess that one's vote, directed in a particular way (differently in each constituency) might contribute to that outcome.