In the previous USD/JPY example, between 2005 and 2006 the U.S. Federal Reserve was aggressively raising interest rates from 2.25% in January to 4.25%, an increase of 200 basis points. During that same time, the Bank of Japan sat on its hands and left interest rates at zero. Therefore, the spread between U.S. and Japanese interest rates grew from 2.25% (2.25% - 0%) to 4.25% (4.25% - 0%). This is what we call an expanding interest rate spread.
In forex, investors use leverage to profit from the fluctuations in exchange rates between two different countries. The leverage that is achievable in the forex market is one of the highest that investors can obtain. Leverage is activated through a loan that is provided to an investor by the broker that is handling the investor’s or trader’s forex account.