In this case it seems to have been much more complex than that. There was UST, which was supposed to be stable, and a linked coin Luna which was a form of balan

In this case it seems to have been much more complex than that. There was UST, which was supposed to be stable, and a linked coin Luna which was a form of balancing token, with smart contracts allowing conversion between the two, with the Luna exchange rate supplied by an oracle. If UST starts to lose value, buy pressure is created because someone could buy 100 of them at (say) $39 and immediately convert to $100 of Luna. If it starts to rise, say to $1.10, you use Luna to convert to UST at a discount and then sell it to turn a profit. There were multiple other mechanisms to this, including one that involved staking your UST for interest and getting Anchor tokens, so that others could take loans. Anchor would give a 20% interest rate(!) and then there was something about loan defaults and buying collateral at a discount and ... Massively complex, and one of those things that works fine, until one day it doesn't.