Raising calories gradually after a diet, in small weekly steps, to return to maintenance without regaining fat quickly.
After a long deficit, appetite is high and dropping straight back to normal eating usually means overshooting. A reverse adds a small amount each week, commonly 50 to 150 kcal, holding weight roughly stable while intake climbs back toward maintenance over six to twelve weeks.
The strong claims made for it, that it permanently raises metabolism, are not supported. What it does reliably is give a structured exit from a diet and keep the client engaged during the phase where most fat regain happens.
Nutrition sits outside a trainer's scope in many places, so know the local rules. The training side is unambiguous: keep lifting, keep the intensity, and expect strength to come back quickly as food goes up.