(1) This section applies if—
(a) a direction is given under section 667, 668, 669 or 670 in respect of a child or young person; and
(b) the child or young person is currently serving another sentence of detention in the youth justice custodial centre.
(2) The Youth Parole Board must direct that the unexpired portion of the period of detention under the other sentence of detention also be served as imprisonment.