Lets say i have this simplified version of my task workflow:
public class TaskGraph
{
[BsonId]
public BsonObjectId Id

{ get; set; }
public List<Task> Tasks { get; set; }

}

public class Task
{
[BsonId]
public Guid Id

{ get; set; }
public string AssignedTo { get; set; }

// other stuff elided
}

In this case, the following query (where I'm trying to get all tasks assigned to me regardless of which workflow they are a part of):

var db = DBUtils.GetMongoDB("taskpoc");
var queryCtx = db.GetCollection<TaskGraph>("TaskGraphs").AsQueryable();
var tasksAssignedToMe = queryCtx
.SelectMany(tg => tg.Tasks)
.Where(t => t.AssignedTo == "tkrick");

... throws the following exception:
System.NotSupportedException

{"The SelectMany query operator is not supported."}

This feature is to enable this to return those results.