SBDs are "half-pegged" there is a floor at $1 because you can always use the "convert" feature to get 1 SBD worth of Steem regardless of market price.

There is no mechanism for keeping the price from rising except that when people receive rewards it prints more SBD which increases the supply and puts downward pressure on the price.

So it will never go below a dollar and when it goes high, the price will eventually trickle back down as people start taking 50/50 rewards instead of just powering up.

Hope that helps.