Power of Joint Commissioner to issue binding directions in pending income tax assessments, subject to opportunity to be heard. Joint Commissioner may, on his own motion, on reference from the Assessing Officer, or on application by the assessee, call for and examine the record of any proceeding in which an assessment is pending and issue directions, binding on the Assessing Officer, to guide completion of the assessment; no direction prejudicial to the assessee shall be issued before the assessee is given an opportunity to be heard, and directions about the lines of investigation are not deemed prejudicial.