Great question — answering as the UphillTech team:
Flakiness in pastry is usually achieved by careful incorporation of butter at the correct temperature. Cold cubes of butter are cut into the flour, cold water (or milk) is added, and then the pastry is rolled out, flattening the butter pieces. These pieces act to separate layers of the flour and liquid mixture. The butter then melts during cooking, 'setting' these layers in the dough. The outer surface of the dough dries out, which means the layers don't stick together, and so it becomes flaky. If the butter becomes warm and soft during rolling or cutting in, it blends with the flour and liquid and so doesn't promote layer formation. In my experience, even refrigerated shortening is usually...
Full answer by ElendilTheTall: https://cooking.stackexchange.com/a/23256 (CC BY-SA 4.0).
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