(a) Except as provided in subdivision (b), the trustee of a revocable trust shall follow any written direction acceptable to the trustee given from time to time (1) by the person then having the power to revoke the trust or the part thereof with respect to which the direction is given or (2) by the person to whom the settlor delegates the right to direct the trustee.
(b) If a written direction given under subdivision (a) would have the effect of modifying the trust, the trustee has no duty to follow the direction unless it complies with the requirements for modifying the trust.