For single engine training flights, I generally do not brief, unless it's an element of the training itself (like briefing your passengers). I do though, be sure to regularly ask of there are any questions during the training, and I pay my attention to the candidate, and wait for an answer (so they know that they should). On the other hand, and learned the hard way, when I am flight testing or multi engine flying with a second pilot, I do brief, and often it's fairly comprehensive as to who will do what when - and who will not! The core element being: "we're flight testing, in accordance with a flight test plan, and I will fly. I will fly no matter what. If you want to take control, say "I have control", and I will let go. But be careful what you ask for, because you'll suddenly be flying.". There will be more, describing the testing being flown, and limitations, and expected exceedances. In one case, after this briefing (and for an airplane I had not yet flown (Caravan floatplane), my right seater told me that he would not be saying "I have control" - because the airplane had no controls on the right side. We agreed that I would fly the whole flight.
I ride jumpseat in the turbine DC-3 a fair amount these days to witness systems flight testing. I'm fairly impressed by the company procedures for briefing, and really like the "are there any questions" part. Briefing is on thing, but if the other pilot (particularly if they will be flying) has questions, which have gone unanswered, that's not good! A few times, while PF in the helicopter, what the PNF was imagining (particularly about an unusual approach and landing) was apparently different than what I was thinking it would be. They only realized the difference in thinking when they witnessed me diverge from the flight path they had imagined. "Are there any questions" would probably been well placed much earlier!
Often, when I ask during testing or a lull in the training, "are there any questions?", I am told "no". Generally, I will pause what we're doing, and say :well, there should be!". in the hope that the candidate will imagine up something to ask. There's no way my instruction is so good, that they got everything!