Whenever you make a forex trade, you're selling one currency and buying another (i.e., if you go long on the USD/JPY pair you're buying dollars and selling yen). You receive the leveraged interest on the currency you buy (the dollar in our example) and pay leveraged interest on the currency you sell (the yen).
If the short-term rate for the currency on which you're earning interest is greater than the short-rate for the currency on which you're paying interest, then you have a net gain just on the interest rate differential. The interest rate differential between the USD and JPY is currently a little less than 4.5 percentage points (i.e., the USD has a higher short-term rate), which would net you around $12 per day on a $100,000 lot. A currency position taken primarily to earn the interest rate differential is called an interest carry trade. Interest rate differentials are reflected in the difference between currency futures prices and spot forex prices (called the premium or cost of carry).
A google search on "interest rate differential" or "interest carry trade" will give you a lot more information on this topic.
Regards,