Ann: Strategic Sale of Shares, page-71

You hold (or buy) shares and then lend them out for a fee, with the current price as the price at which you will get them back. In the meantime you earn the fee so you earn a return on the shares. The other party sells the shares in the hope they can buy them back at a lower price and return the shares to you at the fixed price with a profit. The seller earns a fee and gets the share back at the price at which they sold them. If the share price rises they still own the share and also earn the fee. The shorter is paying a fee and hopes the share price falls by more. They have different views of where the share price will move.

According to the ASX list for 3 October there are only around 700000 PET shares shorted out of 560 million, so hardly major (as yet).