It's not from limiting the supply. It's from agreeing to honor conversions at a rate of 1 SBD = 1 USD worth of steem. Simply put, if the price goes down, one could buy a lot of SBD, convert to steem, and buy back SBD. The catch is that there is a 3 day delay on conversions, but it doesn't really matter if the price of steem isn't increasing like crazy. (If the price decreases, it's better for the conversion)
The other mechanism is that witnesses can set interest rates to encourage holding SBD. But now that I think about it... I'm not really sure how that works since that seems to increase the supply too. (Or perhaps it comes at the expense of the rewards pool). I'll have to revisit that part.