SafeDollar Bonds (SDB) help to incentivize changes in SDO supply during both epoch expansion and contraction periods. For starters, the exchange rate for SDO to SDB is 1:1, but the SDB to SDO ratio is dependent upon the mechanism as described here.
When SDO's TWAP falls below 1 $USD peg, SDB gets issued and can be bought with SDO at its prevailing price. Doing so takes SDO out of its circulating supply.
Contrary to early algorithmic, seigniorage stable coin protocols, SDB does not have expiration dates. All holders are able to redeem their SDB for SDO tokens as long as the Treasury has a positive SDO balance, which typically happens when the protocol is in epoch expansion.