I haven't been able to find a clearly written article on this either. To create propulsion, power must be pumped into the tether. Normally, in the case of a tether used for, like, space station stationkeeping, this power would be provided by the solar panels on the station. I have not seen this specified for this experiment, and am unclear if they are using some other technique I'm not familiar with.

It's worth noting that the Mir was going to be rescued by such a propulsive tether system. Check this out.

I have heard from some sources that the tether equipment, ready to go, was tied up in export restriction red tape while Mir slowly fell back to earth past the point of no return. Dark hints followed that the U.S. Government apparently did not want Mir to be rescued. I have found no independent confirmation of this.