Instead of pivoting like a chop saw or sliding like a slide miter or radial arm saw, it pivots in line with an overhead line shaft—a configuration that allows the blade to be powered by a leather drive belt.
In the days before small electric motors, line shafts made it possible for a single power source to drive all of the machines in a shop. In the early days it would have been done with a steam engine or water wheel. But by the early years of the last century line shafts were more likely to be driven by large electric motors. Read more.