Actually, it's not really a secret at all. In fact, bankers have been doing this for over a hundred years.
Bankers make money by borrowing at low interest rates, then lending at higher interest rates.
We are in a unique time in history in that real estate prices are rising, yet interest rates are dropping. The existing loan remains in place on the investment property, and a new loan is created which wraps around the existing loan. Susie makes a profit on both an interest rate spread and a markup on the purchase price. When the new buyer pays off the remaining balance, Susie pays off the underlying loan. In the meantime, she makes monthly cash flow on the spread between the interest she pays and the interest she collects.