This means that points just within the permiter will be affected strongly and those just outside not at all. This would lead to a visible border, shaped as a sphere. A value for Falloff of 0 would mean that the force would be 1.0 for all points within the black hole, since any number larger 0 raised to the power of 0 is 1.0. The magnitude of the movement of the point is determined basically by the value of force after scaling. We'll consider scaling later.
Lets take an example.
Suppose we have a black hole of radius 2.0 and a point that is exactly 1.0 units from the center. That means it is exactly half-way to the center and that c would be 0.5. What this means is that we must move the point by 0.25 of its distance from the center. The Strength gives you a bit more control over how much a point is perturbed by the black hole. If you set Strength to 0.5, for example, all points within the black hole will be moved by only half as much as they would have been.