Bad money drives out good – but only at a forced rate.
When two kinds of money circulate at a legally fixed ratio, the more valuable one leaves circulation: debtors pay with the overvalued kind and hoard the other. Without a fixed rate the law does not hold, and the better money wins instead.
Useful: The forced exchange rate is the condition, not a footnote – remove it and the effect reverses.
Source: de.wikipedia.org