At Ameritrade, I can "reserve" shares to short by placing an order to short at some limit price. At that point, the shares seem to be mine to short and I can adjust the selling price throughout the day. Perhaps IB works the same way.
a large player could use/allow to be used, his shares to short while garnering a position, but when he is 'full', he can remove the shares from borrowing....
anyone can do this, but bigger players do it in order to stabilize prices LOW while buying