What I am looking for is there must be a clear definition of this practice other than to just help prepare FO's for command.
Not sure what you mean by a clear definition. The procedures should be clearly stated in the company FCOM. For example, the introductory blurb in our Boeing FCOM states the following:
"The FO, when pilot flying, performs the duties listed under PF and the Capt performs those duties listed under PNF. A small number of items in the procedures require specific Capt or FO action or response and are annotated C,F on the particular item in the flow pattern or checklist.
The Commander, however, retains final authority for all actions directed and performed.
The FCOM then clearly states the PF/PNF duties for each phase of flight.