you borrow shares from a permabull and immediately sell them to whatever is buying

all you owe is the number of shares you sold, the original owner doesnt care what happened as long as they get identical ones back eventually. In the meantime, you pay interest on the initial value of what you borrowed and sold

You just sit on the cash

later when the shares are cheaper, you buy shares on the open market and give them back to the person you borrowed from

whatever cash is leftover from rebuying is your profit