A "Workback Workflow" is a type of Dynamic Duration workflow where instead of all task dates calculated and flowing forward from the project start date, rather they all calculate and flow backward from the project end date. This is great for events and other projects where you need to hit a specific target date and want all task dates to be automatically calculated/shifted/updated so that you’ll always hit that target.
To create and use a workback workflow, you'll follow basically the same set of steps for creating a Dynamic Duration template, and then projects from that template, as outlined here:
There are two crucial differences from that document for workback workflows:
In Step 3 of "Create Your Workflow Template", when you select the project to use as the template, you'll see there is a new checkbox below it labeled "This is a Workback workflow (dependencies flow backward from the project end date)". Be sure to check that checkbox.
In Step 2 of "Use Your Template", instead of filling in a project start and due date, you'll only enter a project due date. This is the key ending date which everything will calculate back from. Flowsana will assign this ending date as the due date for the bottom task in the project, and it will work up from there.
Also, you can still use lag times in a Workback Workflow. However, conceptually they work a bit differently: if you want to build in a gap between the start of a task and the end of its dependent task (i.e. the task above it), use a negative number for Lag Time.