With cfd's you earn interest on short position, so you get paid to sit on a short position that goes sideways. The interest earned is 2% below the RBA cash rate. With an amount of 500,000 thats enough to pay the mortgage, plus the prospect of covering at a lower price.
Really though, this would be used to offset the interest paid on leveraged long positions
Great idea in theory, but if the position turns against you, you need enough in your account to cover the 'market to market' amount each day