i think she is talking about during moments with big feelings involved where children aren't going to be able to focus on what you're telling them. the brain is already in the distress state and the child is not going to be able to re-route their neurochemicals (most adults can't, either, actually
). when child is in the distressed state is not the time to try and engage them in any sort of learning. really its more of a learning moment for the parent of how they can help their child in the future.
for instance, last time we were at the store H picked out chocolate teddy grahams. this time, i picked out cinnamon teddy grahams for him because they have less sugar. but i know from experience that if he asks for bear crackers and doesn't get the ones he had before, he will enter into distress tantrum mode - he will be confused, frustrated, and overwhelmed. so when we were at the checkout, i showed him the box and said "I got you some more bear crackers. But look, these are NOT chocolate, okay? They're cinnamon."
of course i can't guarantee that he will be ok with that when the time comes, but explaining to him beforehand, i've learned, helps him handle the situation better when it comes time.
if i were to wait until he was already having a meltdown because they weren't the right crackers, it would be pointless to try and use that as a teaching moment for him. i've already used experiences similar to this to teach myself that something has to be done beforehand to help him and better prepare him to handle the situation.