The Rate Tracker home loan offered by BankWest has been the main driver behind the increase in the bank's residential mortgage book, according to Paul Vivian, head of mortgages and savings at BankWest.The Rate Tracker variable is one per cent below the major four bank average standard variable for the first two years of the loan, before reverting to the ongoing variable.Vivian said the new product is now the marque mortgage product for the bank, accounting for more than half of all new loans.Residential lending by banks in Australia increased by $22.8 billion in the quarter to May 2008, according to APRA statistics, with BankWest accounting for 5.6 per cent of this increase.This is an improvement compared to the February 2008 and November 2007 quarters, where the bank net loan growth was around $600 million.Vivian would not be drawn on whether the majority of new loans are through the direct channels, or the ever-increasing branch network, or brokers, but aggregators Australia Finance Group and eChoice are experiencing strong customer demand for the product.Mark Hewitt, general manager sales and operations at AFG, said the introductory style BankWest Rate Tracker is currently a very popular product."BankWest are now up to twelve or 13 per cent of monthly volume, and in the past their share was well below ten per cent. A fair proportion of that will be made up of the Rate Tracker product."AFG loans at introductory rates accounted for 13 per cent of total loans in May and June, up from ten per cent in April and more than double any of the twelve months to March 2008.Michael Peters, chief executive officer for Victorian-based aggregator eChoice, said borrowers are generally heading into variable loans due to unattractive term loan rates - with borrowers finding the BankWest Rate Tracker to be a product of choice."The BankWests and INGs get in our environment as much market share, if not a little more, than the majors."