Terms of trade is the cleanest fundamental link between commodity prices and currencies. An economy exporting iron ore and importing manufactured goods sees national income rise when ore prices rise, independently of how much it ships, and the currency tends to follow.
This is the mechanism behind commodity-currency behaviour and behind petrocurrency moves around oil. It also explains the asymmetry between exporters and importers: the same oil rally supports one currency and weighs on the one whose economy buys the oil.
Under a free-float the exchange rate absorbs part of the shock automatically, which is why floating commodity exporters have smoother domestic cycles than pegged ones.
Example: export prices rise 12% while import prices rise 2%. Terms of trade improve by about 9.8%, so the same export volume funds nearly a tenth more imports than a year earlier.
Related: commodity-currency, petrocurrency, current-account, free-float