This option will help to manage the workload when you have fixed deadlines and labor units for each task and they should remain fixed even if you assign a task.
To enable it, open Project settings and select Fixed duration and estimation 1 in Workload calculations. GanttPRO will distribute working hours between resources.
Suppose that a task duration is 2 days and estimation is set to 8 hours. With fixed duration and estimation, those parameters will not change no matter how many people you assign to the task (examples 2 or 3) and what their working schedule is.