One of the most confusing misconception in project management, is to think about tasks in therms of time duration. The true unit of measure for a task should be work hours and not just hours. When a manager thinks of a task duration, he somehow thinks also of how many resources should be involved in completing the task. A project management software should formulate somehow this approach and should deliver tools to deal with it.
When assigning resources to a task for the first time, the task behaves as if it is set as fixed duration, meaning that the more resources you add the more work accumulates for the task.
When this first phase is finished the task becomes fixed work, meaning that the total work for it will become fixed.
That will shape its behaviour in such a way that when you add more resources, for example, either the duration (not the work hours) will shrink, or the resource units will shrink (they will become used less than 100%).
One of the two scenario is applied, function of which settings are applied to the task. If the task is set as fixed units, the duration will decrease, if the task is set as fixed duration, the usage percentation of resources will decrease.