Life threatening or as said in CAA guidance "you have control" from now defunct P1 only I'd say?
unless you assume the responsibilities of PIC for the remainder of the flight.
So there are the two slightly different definitions already!
The P1 might just feel like he needs a rest, iaw the terms of his medical limitation and the CAA's guidance notes on the matter of "safety pilots" doesn't specify apart from the words "have to". Once taken over as P1, could control be handed back to the original P1 for the remainder of the flight time? No guidance is given.
There have been a number of previous discussion about the situation where two qualified pilots sit side by in an aircraft certificated for singe pilot operation, as most, if not all light aircraft are. Both might wish to claim & log P1 for part of the flight.
Question is, what's the difference? Can a qualified pilot claim P1 time flown from the "wrong" seat, or not, if he handles the controls in flight where he hasn't signed the tech log, as in the case of medical incapacitation of the person who has signed?