With Your bathroom door closed or house door if it’s your kitchen fan a somewhat high velocity air will be sucked in under the small opening at the bottom of the door.
As you decrease the pressure inside of a room and draw air out and as you decrease the size of an opening the velocity of the air moving will increase.
This explains why if the bathroom door is wide open you do not feel any air moving through the door and hitting you in the face because of the large opening
But if you have a small opening because the bathroom fan only draws this particular one 70 CFM you will have a high velocity trying to force all that volume of air through a small opening and you can easily feel it and it will easily be able to move a piece of toilet paper
