Cold fat creates steam, thus flaky layers. According to food scientist Shirley Corriher, cold fat melts slowly in the hot oven, giving the dough a chance set on either side of it. When the fat does start to heat up and melt, the resulting steam puffs layers apart, creating flakiness. Shortening, with its higher melting point, helps the fat stay firmer longer. Cold liquid stimulates less gluten formation.
Strong gluten is good for bread, but it makes pie crusts tough.
Add a little lemon juice to the dough, too, because the acid can help shorten the gluten as well. Use a food processor for quick, hands-free mixing. The goal is to quickly combine ingredients and cut in the fat with a brief pulsing, just until the mixture is crumbly. Parchment is key to fearless rolling Fear of rolling seems to be the main complaint I hear from students in my classes. Roll from the center out, turning the paper clockwise a little after each roll.