OP
TPES
As I understand it, positive pressure fans take air from the loft and blow it down through a hole in the ceiling into the landing/hallway. This recovers a little heat from the loft and, by slightly pressurising the house, stops cold draughts at doors and windows. I guess there's also a filter to stop dust from the loft being blown into the house.
Edit, as he says above ^^^^^^^
I've been doing 2 a day for the past couple of years. I won't do 3 a day