I think the filter cut off frequency is controlled by varying the 4016 gates' resistance with a portion of the main bbd clock. Lowering its frequency will increase bbd delay, with a corresponding drop in filter cut off frequency to preserve as clean a signal as possible. MXR used this trick on several of their products too, but I have not seen it used by Roland/Boss before. Ah yes it was used in the original ADA Final Phase unit, although I haven't seen the circuit for the reissue.