TSQL LAG equivalent

I am trying to understand how LAG works, by converting the following code back to using self-join sql code without using LAG.

http://ift.tt/1DdiSOV

USE AdventureWorks2012;
GO
SELECT TerritoryName, BusinessEntityID, SalesYTD, 
       LAG (SalesYTD, 1, 0) OVER (PARTITION BY TerritoryName ORDER BY SalesYTD DESC) AS PrevRepSales
FROM Sales.vSalesPerson
WHERE TerritoryName IN (N'Northwest', N'Canada') 
ORDER BY TerritoryName;

Any suggestions please?