The mechanics are that you loan shares, sell them at the price you think is too high, buy them at the lower price, then hand those cheaper shares back.

The argument for doing so is that humans err in favour of optimism -- ie, that prices will rise. But you can make money on the market moving either way.

Looking on it as a total outsider with limited knowledge, it seems like a fascinating niche in the financial world. But I imagine it is also even more stressful than betting on the price rise, because you're more often going to be outside the pack.