When making pie crust, time is of essence. Be efficient and fast as you cut the margarine and butter, and every step after. As the margarine / butter melts, it creates air pockets in the pie crust leading to a crisps crust. So you need to work fast to minimize the time where the butter pockets already melted before you even started baking. Cutting the margarine into thick cubes as described in the text of this article (instead of using forks or a butter cutter) also prolongs the time the margarine stays unmelted.