An investor can profit from the difference between two interest rates in two different economies by buying the currency with the higher interest rate and shorting the currency with the lower interest rate. Prior to the 2008 financial crisis, it was very common to short the Japanese yen (JPY) and buy British pounds (GBP) because the interest rate differential was very large. This strategy is sometimes referred to as a "carry trade."
The broker basically resets the positions and provides either a credit or debit for the interest rate differential between the two currencies in the pairs being held. The trade carries on and the trader doesn't need to deliver or settle the transaction. When the trade is closed the trader realizes their profit or loss based on their original transaction price and the price they closed the trade at. The rollover credits or debits could either add to this gain or detract from it.
For traders—especially those with limited funds—day trading or swing trading in small amounts is easier in the forex market than other markets. For those with longer-term horizons and larger funds, long-term fundamentals-based trading or a carry trade can be profitable. A focus on understanding the macroeconomic fundamentals driving currency values and experience with technical analysis will help new forex traders to become more profitable. (For related reading, see "Benefits & Risks of Trading Forex with Bitcoin")
Interesting. Rob is one high-profile forex 'guru' I've been loosely following for several years, and he seems to flit from system to system. For example, a few years back he was very enthusiastic about a system he called the Hopper (which was little more than a MACD crossover) which he was touting with his ladyfriend Jennifer Thornburg (who likes to write articles about Sex and Trading, btw). More recently Rob's been promoting EAs, including some that take profit quickly while allowing floating losses to snowball. Given all of this, I've never bought anything that he's been offering, and I've found it difficult to take him completely seriously.