In this case I interpret "backed by" as that the central bank will give you the corresponding amount of gold for your currency. But, the opposite would not be t

In this case I interpret "backed by" as that the central bank will give you the corresponding amount of gold for your currency. But, the opposite would not be true, they won't give you currency for gold alone, you'd have to also provide the same kind of backing as for non gold backed currency. So paid in gold or the currency, you'd get the same number of apples in each case. But if you'd convert the currency to gold via the central bank (rather than on the open market) you'd get 1/5 as many apples in the second case. In normal operations this won't make any difference, no one in their right mind would use the central bank rather than the open market to buy gold. But it means that the central bank can't just print money as they see fit, reducing the risk of runaway inflation. It also acts as insurance in the case of runaway inflation, you'll lose at most 80% of the value.