An FX forward is used to fix in advance the yen amount of foreign currency to be paid for imports or received for exports. The delivery date is set either as a single fixed date (fixed-date delivery) or as any day within a set period (period delivery). A forward generally cannot be canceled, and even if the rate on the delivery date is more favorable than the contract rate, the currencies are exchanged at the contract rate.
Consider a hypothetical example in which you agree to buy USD 1 million in 6 months at 148 yen per dollar. Your payment is fixed at 148 million yen. If the rate on the delivery date is 160 yen, you pay 12 million yen less than you would without the forward, and at 135 yen you pay 13 million yen more.