An uncontrolled descent or ascent of a counterweight fly system's arbor and corresponding batten, caused by mis-management of either load or counterweight. The system's rope lock should not be required to hold an imbalance of more than 75 pounds. In the event of a runaway, DO NOT attempt to stop it--take immediate cover, as far away from the lockrail and batten as possible. To quote punktech: "Runaway" both a suggestion as to what you should do and a description of the situation.
For more information, see "Counterweight Questions Answered," written by Bill Sapsis, on J.R.Clancy's site.
For more information, see "Counterweight Questions Answered," written by Bill Sapsis, on J.R.Clancy's site.