In a TARF under which the holder buys dollars, each time the exchange rate on an observation date is weaker for the yen than the strike, the difference is added up as the gain per dollar, and the transaction ends the moment the total reaches or exceeds the target.
When the rate is on the unfavorable side, the obligation to keep trading at the set rate remains, so the gain is capped at the target while the loss has no ceiling. Some TARFs include a condition that raises the amount exchanged on the unfavorable side (leverage). Some FX TARFs sold to investors in Hong Kong had this condition, which made their losses even larger.