This option, in the multiple-resource setting area of the schedule editing dialog, will result in the different resources being scheduled at different times, if the scheduler determines that doing so ensures that the task can be finished sooner. How the estimated effort for the task is divided among the resources is determined by the sub-options:
Divide effort for earliest finish - selecting this option will cause the scheduler to find the division of work that yields the earliest finish time for the task. For example, consider a task whose estimated effort is 3 days, with two resources A and B. Assume that the task should start today, and that B is already assigned to a different task for today, but is available tomorrow, and that A is available both today and tomorrow. Then the scheduler will assign 2 days of effort to A, scheduled today and tomorrow, and one day of effort to B, scheduled for tomorrow.
Divide effort equally - selecting this option will cause the scheduler to assign equal portions of the estimated effort for the task to all the resources. In the example above, each of A and B would be assigned 1.5 days of work, and in A's case the work would be scheduled for today and half of tomorrow; in B's case, the work would be scheduled for tomorrow and half of the day after.