Glycaemic Dip
A glycaemic dip is a fall in blood sugar below where it started, typically two to three hours after a meal. It is not the spike the popular story blames, and it is not the clinical condition described at reactive hypoglycaemia. Across a large group, a bigger dip goes with slightly more hunger and slightly more eating afterwards. The relationship is real, consistent across that group, and weak.
What it is
Lunch at one o'clock, dip at around three or four. That is the scene the folk story is trying to explain, and the timing evidence points at the dip rather than at the rise that came before it, which is the opposite of what the popular version says.
The distinction from the clinical condition matters and is easy to lose. A dip below your own starting point is not the same thing as blood sugar actually falling below the normal range, which is what the clinical term means.