The stale and humid air from wet rooms, bathrooms, kitchens, etc. is continuously extracted at design rates through ducting and is drawn in to the MVHR unit. The heat from the extracted air is transferred to fresh incoming air through the heat exchanger in the MVHR unit.
Once passed through the heat exchanger within the unit, the warmed, clean, fresh, filtered air is distributed around the home through a separate series of ducts. The efficiency rate of an MVHR system is 90 – 95%, which means you can recover 90 – 95% of the heat that would otherwise leave the building in the extract air.
If you were simply relying upon trickle vents for ventilation, you would recover none of this heat and it would all be completely wasted.