Oh dear me! To say this topic has been done to death a hundred times here would be an understatement. Try using the search function, and then spend the next week reading the misinformation, disinformation and misunderstandings that you'll find.
Bt succunctly, PICu/s has nothing, that is NOTHING to do with PF or PNF.
(How could it, if P1 depended on who was PF what would the Captain log when PNF?)
It requires the FO, as stated above, to perform all the functions of P1 for the entire flight without intervention...
That means it cannot be achieved without prior agreement and arragement with the Capt from the first moment of pre-flight planning as the FO clearly has to do ALL of this as P1, make ALL fuel, weather and operational decisions etc and continue to do so until the end of the flight, presumablt including post flight duties.
Logging PICu/s automatically as PF is not only incorrect but fraudulent as the basic requirements above have clearly not been met. Simply put, Parker Pen time.
That said, it seems to be what most people do, but that doesn't make it right. The UK CAA seem to turn a blind eye too, so that really renders it pointless as a valuable exercise in practicing command skills and turns it into a paperwork fudge to gain an ATPL from the RHS of an airliner.
The worry is if this fudge is employed in smaller craft to allow the pilot to gain his ATPL so as to act as Capt without having had the P1 training experience the regulators have decided is needed.
Pity.