They certainly used to in some cases, very few people actually did this so it wasn't a huge issue. The money is just lodged and does indeed just sit there, but while sitting there you are being charged interest on the net balance so it saves you interest. Now if you are on a low tracker then you may be able to get a higher rate of interest elsewhere but if not then it can be of benefit. The actual offset mortgages actually worked slightly differently in that the money offset was held in a totally separate account to the mortgage account.