Your question is interesting, since it provides us with the chance to give a more detailed explanation of how this rate applies.
The rate has two prices that apply depending on the outdoor temperature. Most of the time, the price is lower than that of the rate D; however, during cold periods, when the temperature is below or equal to -12 C or -15 C, depending on where you live, the price is higher. Thus, since these temperatures are generally reached only a few days per year, a customer with the rate DT saves on his annual energy bill, even if a part of his electricity consumption is billed at the higher Rate DT price and he purchases fuel to heat his home during very cold weather.
To maximize savings, the customer must, during very cold weather, be sure to heat his home with fuel and also to reduce his electricity consumption, for example, by decreasing hot water usage and not running electrical appliances so as to avoid, to the extent possible, being billed at the higher Rate DT price.