The key word is "fungibility"

as in, you give back _a_ share not the same share.

So you buy a bunch of shares at x price, you agree to hand them back in n days time.

You make money by selling the shares immediately and then you buy shares later at a lower price, then when you hand back the shares, the profit is the difference between ho much you sold them for, and how much you bought them back again.

The risk is, you _have_ to give the shares back usually at a fixed point in time. So if the price rises, you have to pay the difference. (there is normally a fee as well, to borrow the shares.)