If you have a copy of Shulgin's "TIHKaL" to hand it would be worth reading chapter 13, "Flashbacks". In summary, flashbacks can occur as the result of any particularly intense emotional experience, whether that be flying a light aircraft for the first time, scaling a mountain, experiencing an earthquake - or indeed, having a peak psychedelic experience. The inducer of the flashback to whatever type of emotional experience will consist of some set of relevant sensory cues such as specific sounds/music, scents, or a particular location. The flashback itself will consist of some set of sensory or emotional impressions in recollection of the initial experience. This need not entail any loss of self-control.
Shulgin's own example was one where he hurried to a concert during a snowstorm. Snow had accumulated on his collar and proceeded to melt during the concert. He was greatly surprised as the meltwater trickled down the back of his neck. Many months later on a warm, sunny summer's day when he heard the same piece of music that had been playing at that previous time, he was so convinced by the sensation of icy water trickling down his neck that he reached up to check the back of his neck with his hand. (Of course there was no water there!)
Well, that's Shulgin's story anyhow; but the thing is, flashbacks are not unique to psychedelic drugs - they are a normal function of memory and the demonisation of psychedelic flashbacks is yet another instance of anti-psychedelic propaganda. It is profoundly unhelpful to condition people to respond with anxiety or even panic to a perfectly normal aspect of mental function.
Here's an idea if any of your partners remain concerned about reactivation after these assurances - if you choose to have a musical soundtrack during the experience it may be helpful to pick a playlist that excludes anything that has the remotest possibility of being heard on any mainstream radio channel. This will help preclude reactivation through a musical cue while driving.
The decision to take a psychedelic drug and, more to the point, the actual act of taking it is like anything else where there is no turning back - stepping off the edge during any kind of daredevil leap, going after that love interest, ditching that boring job in no uncertain terms: these are all daunting things which require courage. And I certainly required a bit of courage when it finally came to adding mimosa tincture to my Syrian rue brew after a good few weeks working with the rue alone.
Boldness will typically be rewarded and suitable preparation increases the chances of that reward. So, sharing awareness of what those preparations might be seems key to optimising the outcomes for your prospective partners in this situation. Do you have a good picture of what kinds of preparation you might advise?
“There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli