Can you explain why a simple ball in a basement drain can keep water from coming into a house?

Well it won't unless it is a sewer floor drain and the ball is driven into the drain opening. If the back up is your sewer and you plug the floor the water is still entering the house but it is going up inside the drain pipe to the next lowest fixture, it may eventually come out of a toilet somewhere.

Can you explain why a simple ball in a basement drain can keep water from coming into a house?
If you're asking about a floor drain Flood Guard perhaps this will help. Regards, Tom