The Bank of Canada maintains a number of facilities, and it recently announced the creation of a new facility, by which they provide liquidity; so they lend, essentially, on the basis of security posted by financial institutions and other investors. This is done very much on a demand basis. So the bank will say they're willing to lend up to x amount, the rates are established by a competitive auction process, and the amount that goes out the door is a function both of the limit the bank is willing to establish and how much institutions are interested in drawing on these facilities.
On March 3rd, 2009. See this statement in context.