Well technically SBD (STEEM itself not at all) is somewhat pegged to the US Dollar. It works pretty well if the SBD is under 1 USD, but it does not really peg if the SBD is over1 USD. It is not nearly as stable as tether.
The way the peg works, is that e.g. for your post rewards, you can choose to get SBD paid out, and if you do, you will get these SBD expecting that the SBD price is 1 USD, that means, that somewhat pressures the price in the right direction. But the real part about the peg, is the convert function. You can always, at any time convert any amount of SBD to STEEM. That takes one week where you can neither get back you SBD nor make use of your yet to come STEEM. After these 7 days, you get as much STEEM as you would if the SBD was 1 USD (using the feed price the witnesses provide)
That means if e.g. the real price of SBD in the market is 0.10 USD, and the STEEM price the witnesses provide is 1 USD, and you start a convert of 100 SBD, and prices don't change of the convert period, you will get 100 STEEM. If you would have converted them in the steemit market, you would've only got 10 STEEM out of your SBD. And that of course pulls the SBD price up.
I hope this was a comprehensible explanation ;-)
If you have any questions, please don't hesitate :-)