Pea-sized fat creates flaky layers through lamination
When fat (butter or shortening) is left in pea-sized pieces and mixed minimally into flour, each pocket becomes a thin layer of fat surrounded by flour. During rolling and folding, these pockets are stretched and stacked. In the oven, fat melts and water in the fat evaporates, creating steam that puffs the flour layers apart, producing the characteristic flaky texture.
The size matters. Pea-sized pieces stay discrete during mixing, creating distinct layers. Smaller pieces (pebble-sized or smaller) blend too completely into flour, distributing fat throughout instead of creating pockets. This produces a tender crumb but eliminates the flakiness.
Uniform mixing creates short, tender crust instead
When fat is worked thoroughly into flour (as in a food processor for too long), the mixture becomes uniform, resembling wet sand. This style of crust produces a short, tender, crumbly texture. It breaks apart easily (the 'short' descriptor) but cannot develop layered flakiness. Shortbread and some pie crusts intentionally use this method for a delicate, sandy bite rather than shattered flakes.