You got me curious, so I dug up their patent. This is as close as they come to addressing it.
> Although not shown in the previous figures, the circular mass accelerator structure 150 may comprise a second exit port directly opposite the exit port 115 to capture the counterweight 135 that is released simultaneously with the launch vehicle 105 to minimize an imbalance on the motor at the time of release. The counterweight 135 may be a solid material, or a liquid such as water.
The use of a liquid is a curious idea. Perhaps it could be dispersed in such a way as to spread the force of the counter weight being released across a wide surface area? Like a small explosive forces the liquid out in all directions?