Shorting - trying to make a profit by selling borrowed shares and buying them back at a lower price - may be an alien concept to many but it is becoming important to understand. Look at Rank.

In the past two weeks, shares in the casino and bingo operator have climbed 23 per cent. Superficially, that rise has been down to stakebuilding by two overseas gaming companies, Genting and Guoco. But that is half the story.