Quite simple. The classic textbook "sliding scale" works this way:
Before eating, you measure your blood sugar. You then administer a bolus of insulin based on that number. Period.
Notice that this does not consider what, or how much, you are going to eat. It doesn't consider your I:C ratio, your ISF, or anything else about the way your body processes food and insulin. Just a fixed number based on your PRE meal blood sugar. In effect, this method assumes that a green salad and a chocolate cake are equal.
Put that way, it sounds almost comical. The reason it isn't at all funny is that for years it was the accepted medical approach. For some people, it still is.