It actually interupts convection....heat naturally rises, so if you have a fan blow it back down or even blow it up on the ceiling the "wind" if you will, will cause the heated air to move along the ceiling till it gets to the wall and then it will travel down the wall, to the cooler air at the floor and it will naturally rise again, and the cycle continues.
WITHOUT the fan, the hot air rises to the ceiling, the cool air drops to the floor, and the air around windows and doors gets cooler (in the winter) and that happens until the entire floor is cold enough to set the thermostat off, then the fan comes on, the heat comes on and keeps going till the thermostat is warm, then it starts all over cooling again.
If your house is open to the second floor this temp change can be extreem it the system isn't set right. The floor of the first floor will be ice cold and up stairs will be smoking hot.
