A Compensator is a reduced voltage motor starter. It's use can have a cushioning effect on the mechanical equipment ( sort of a rudimentary soft start). The compensator may be electrically activated with pushbuttons (auto comp) or manually operated via a long handle on the side ( often called a one armed bandit). The voltage reduction is accomplished with a series of large windings which are switched into the circuit for starting and out of the circuit for across the line running.