It is certainly in the white paper. It is also in the white paper that the interest rate should be raised if the market is valuing SBD below $1 USD and the debt-to-ownership ratio isn't too high. And interest payments should be discontinued if the market is valuing SBD above $1 USD.
Unfortunately (in my opinion) the interest rate has not been responding to the market undervaluing SBD... I don't think it has ever changed from the 10% apr that it has always been. It's an important part of the peg that should be functioning and should also be promoted more than it currently is, so that it has an actual effect on the market rather than being some obscure thing as it currently is.