These buoys were originally designed to facilitate space-goat herding. The weaponized version may pull a space-goat's skeletal frame straight out of its aft sphincter, but the tether has to be significantly more powerful to hold against the thrust of most modern engines.
This ability won't activate unless there's at least one enemy in range to tether.
Tethered ships that try to move too far from the buoy are pulled back. They can still move and act freely within that radius, however.