This is a scheme for transportation between low earth orbit and the lunar surface with almost no propellent expenditure.

The basic architecture is that there would be a tether in an elliptical, equatorial orbit around the earth that would throw a payload to the moon. At the moon a lunar rotovator would catch it and deposit it on the moon.

To keep the system balanced, the rotovator picks a similar mass payload (or a rock) off the moon and chucks it back at the earth.

In theory this system would require no fuel expenditure at all; because the mass is balanced. However, because the moon's orbit is in a slightly different plane than the earth the rotovator and the earth tether do not quite line up; so a small corrective burn is needed, but the overall delta-v needed is massively reduced.

Interestingly, because the Moon is in a high energy orbit around the earth, it is actually possible to generate energy from this system by picking rocks off the moon and dumping them on the Earth.