My pre-takeoff checks always include me saying to the instructor “In the event of an emergency, I shall say ‘You have control.’ “
'Sounds okay to me if saying this is an acknowledgement that upon agreement between the two pilots during an unexpected event, control shall be stated as changed.
On a number of occasions, while conducting a modification design approval test flight, I was flying with a pilot with whom I had never before flown, often more experienced than I, and perhaps in a type I had never flown - certainly at least with a change, subject of the test flight. I would allow that pilot the briefing they desired. My briefing to them (pre entering the aircraft) would include: "If I hear you say that you have control, I will immediately let go, and you'll be flying.". However, If you choose to take control, at that time, I will no longer be conducting the required flight test, and most likely it will be incomplete (meaning a reflight will be required). With only one exception, my flying was never interfered (including two real emergencies). That one exception, the boss and the company Chief Pilot determined in short order that the company pilot they'd sent with me just was not well suited to this kind of flying. I was check out, and completed the rest solo.
A clear understanding of who will fly when is vital - so the discussion is important. Then, adhering to the agreement is pretty important too. For those flying tandem seat aircraft it is critical to state taking control, and other pilot state relinquishing control. I don't get too hung up on the precision of how communication is done, as long as it is clear.