The payoff is the difference in the foreign currency multiplied by the fixed exchange rate, so the exchange rate at expiry does not affect it. In exchange for having no exposure to the exchange rate itself, the premium depends on the correlation between the exchange rate and the price of the underlying asset1.
Consider a hypothetical example in which a call on a stock priced at USD 100 has a strike of USD 100 and a fixed rate of 150 yen per dollar. If the stock is at USD 120 at expiry, the payoff is USD 20 x 150 yen = 3,000 yen per share. The payoff is 3,000 yen whether the exchange rate at expiry is 140 or 160 yen per dollar. When the stock price is above the strike, a dollar stronger than the fixed rate at expiry (the direction that raises the amount in yen) adds nothing to the payoff, and a weaker dollar takes nothing away.
It differs from a foreign-currency option whose payoff is converted at the exchange rate at expiry. The exchange rate is fixed at the time of the trade, which is the feature of a quanto2.