If you have bitcoin loans then you de facto have bitcoin short positions. Investment sites that allow you to short by checking the box for 'short sell' rather than 'buy' is just a convenience.
The mechanism for short selling is to get a loan for say, 10 BTC, then use that to buy cash (say, $650 USD today), then when it's time to pay back the loan you buy 10 BTC at current market rates to pay back the loan. If you pay the loan back a month from now and the price has dropped by $10 per BTC, then it would cost $550 to buy 10 BTC to pay back the loan, earning you $100 USD.
Of course, you have an interest rate on a loan, so if your interest rate on the 10 BTC is, say, 1% then you have to pay back 10.01 BTC, which would eat into your profits from a short position. You want to take this into account when timing your purchase of BTC to cover your short position. If a lot of people start to think that BTC is going to go down in value then demand for short positions will increase, which will increase the demand for BTC loans, which will increase the short term loan interest rate.
How many services are there for BTC loans? In aggregate, how much BTC credit is really out there? If there isn't much BTC credit out there and short selling picks up then it would be very profitable to be a seller of BTC credit, because as demand increases so do interest rates.
Great response.
Thanks