The Financial Post reports in its Friday edition that mortgage rates are near three-year lows, but for many big bank borrowers, falling rates are a double-edged sword. Guest columnist Robert McLister writes falling rates trigger break penalties that banks calculate using "interest rate differentials" (IRDs) -- a term that roughly translates to "math designed to make you cry." These IRD charges are essentially based on fake rates, and cost Canadians billions every year. We could pull several names from a hat as examples, but let us consider Toronto-Dominion Bank. On Wednesday, TD took a chainsaw to its publicly posted rates. Its two-year fixed rate nosedived 195 basis points. If you have the misfortune of breaking a TD mortgage right now, it is possible that rate drop just showed up with a vacuum and emptied your bank account. For a TD borrower breaking a three-year fixed rate with around 29 months to go and a $500,000 balance, that 195-basis-point drop could potentially cause their penalty to go from roughly $5,600 (three months of interest) to well over $17,000, according to Prepayment Penalty Mentor. This little charge is based on the IRD calculation: TD keeps the receipt and weaponizes it the minute you try to leave.