So what is it and how does it help to have a secondary workflow?
To better depict the picture I'll start with the following scenario: there is a complicated document management system with many approval workflows etc. one of many workflows is triggered by a document upload, the workflow type is determined by some of the metadata of the document. when an approval task is created by "collect data from user" action in the SPD workflow creation, the approval task should indicate the type of the document and some additional metadata of the document such as if approval of the document is urgent, due date for the approval, etc. (you finish the list :-) All of this information is indicated by end-user during the document metadata input. But... when the task is created the next step of the workflow is not executed until user completes the task, even if I put "update item" action in the next step to update metadata of the task with document's metadata, it will do so only after the task is completed, which completely defeats the purpose of this step.
Secondary workflow to the rescue…..
When you create a workflow on the tasks list and start it on item creation. You will update the metadata of this task with the metadata of the document and reference the document by using "Tasks:Workflow Item ID"
In the long run as soon as the task gets created it will update itself with the document metadata by finding the doc based on the following criteria "document library: ID" = "tasks: workflow Item ID"
This is applicable in any scenario where you want to reference the item that originated the workflow that had created the task.