Ok, from CAA LASOR's (hope its ok to quote it)
5. PICUS (Pilot-in-command under supervision):
Provided that the method of supervision is acceptable
to the Authority, a Co-pilot may log as PIC flight time
flown as PICUS, when all of the duties and functions
of PIC on that flight were carried out, such that the
intervention of the PIC in the interest of safety was
not required. (For further details please refer to
Co- Pilot paragraph at the end of this Appendix).
search for "picus" in LASORS if you need more clarification