Rewards are given in STEEM, and SBD are created using them as a collateral.

For example, if the reward is 133STEEM and 1STEEM is worth $1USD. The author would receive ≈100STEEM (75% of the reward) wich should be payed in the form of:

• 50SP : They are just locked Steems, for long term investment.

• 50SBD : Because 50STEEMS are worth $50USD, we can use them as a collateral to create 50 tokens and ensure that they will be worth at least $1USD in STEEM.
  That's why they are called Steem Backed Dollars.

• Additionally the author can also decide to get all of his reward in SP, in which case no SBD would be created.

If STEEM goes up, SBD rewards will go up. However, the contrary is not true.

(At least that's how I understand it works, I haven't finished reading the white paper)