It is the "convert" function which is the failsafe to market failures: https://steemit.com/money/@theoretical/why-would-anyone-use-the-conversion-function-when-they-can-trade-their-steem-dollars-on-the-market
"When this trade is possible, market participants with deep pockets should step in and do it, which will drive the price back up toward $1.00 -- it is highly unlikely Steem Dollars will ever trade much below $1.00 for long."