GMm/r^2 and other stuff...
With a small body like an Eros, something that can be orbited gingerly, but can also be leapt off by an average human, it wouldn't hold much of an atmosphere. You could probably treat each mollecule ballistically, or collisionless. Most would be in a shallow hyperbolic orbit past the body. I have the feeling they meant this technique for rather small bodies
If you have a fancy plasma, high-ISP engine and all of your exaust ions get implanted in the surface of the tow-ee, the net effect will be close to zero.