https://steemit.com/steem/@buggedout/impending-steem-dollar-purge-the-1-key-indicator-to-watch#@geekpowered/re-buggedout-impending-steem-dollar-purge-the-1-key-indicator-to-watch-20180625t084607628z

Once the debt level gets to 10% then the convertibility of SBD into $1 USD worth of STEEM is compromised. At this point the system will only convert to a fraction of $1 USD worth of STEEM as the debt level is too high and a “Haircut” is applied to SBD conversions.