If the vessel is a sailboat, with a motor, and the vessel has been set up to perform a mixed sail / motor route, then this option may be useful.
When this option is on and the solver runs, the system will first solve for the solution where the vessel is allowed to both motor and sail. After that solution is completed, if a path has been created, it is analyzed. If the path used the engine, then a second solution is calculated where the engine is not allowed. Comparing these two solutions allows you to see the relative benefit of using the engine.