From a Crema magazine article on espresso preparation…
Heavy conical upper burrs pull the beans down, compressing them until they shatter into smaller fragments to enter the flat burrs, to be sheared into the final grind.
Seventeen grams of the fluff exits the edges of the flat burrs and drops into a chute along the sides of the grinding head. A whirling brass paddle smashes into the coffee, whisking it on a furious circular journey at about 450 rpm until it is forced out a square portal to tumble into the dosing hopper. After grinding, this is the first real assault on our sweet coffee – the impeller smashing it into lumps, bruising the lipids and destroying a little of the fragrance.
Because of the short, pressurized percolation cycle of around 25 seconds, the final consistency of the ground coffee is critical to achieve crema, and preserve the full amount of fragrance the bean has to offer. The flat burrs shear the bean into a complex consistency that looks like snowflakes under a microscope. To accomplish this the flat burrs must remain very sharp and require changing every 500 pounds. The goal of the grind is to achieve the highest surface area of exposed aromatic oils, lipids and sugars to be transported quickly by the brewing water into your cup. The rapid percolation cycle and pressure are the unique characteristics of the espresso method that allow us to preserve the most delicate fragrance through the brewing process.