Reverse Resource Allocation is generally explained in one of two ways:
1. If your project has a fixed end-date (regardless of resources needed), then you will very likely build your project schedule backwards. That means you start with the last task on the last day and work your way towards the present. In the same way you will also be allocating your resources in reverse. In this way you can determine on which start dates you will need the resources in question.
2. Reverse Resource Allocation is sometimes also defined as a technique to schedule specif, expert resources onto your project. Let's assume that you are creating a highly technical product, that there is only 1 person in your company who has the specific expertise to work on this and that this person must work on this task on January 20th. You now begin scheduling and assigning resources from this point backward so that you can assign this resource at the correct time.