A floor with a strike of 0% is a right to receive only the amount by which prices fall. It is sometimes embedded in a contract, as with the lower limit on the coupon of an inflation-linked bond or the principal guarantee on Japanese inflation-linked bonds.
Consider a hypothetical example of a zero-coupon floor with a notional amount of 10 billion yen, a term of 5 years and a strike of 0%. If the price index falls from 100 to 95 after 5 years the payoff is 500 million yen, and if it rises to 105 the payoff is 0 yen. If prices do not fall, the buyer loses the premium paid.
It is the reverse of an inflation cap in that it pays the amount below the strike, and the strike can also be set at a negative value.