Rubbing in
Rubbing in is the technique of working cold fat into flour with the fingertips until the mixture resembles fine breadcrumbs, which coats the flour in fat and limits gluten development for a tender, crumbly pastry.
Use fat straight from the fridge and lift the mixture as you work, letting it fall back into the bowl to keep it cool and aerated. Stop while a few pea-sized lumps of fat remain: slightly uneven fat gives a shorter, flakier result than a perfectly uniform crumb.
Warm hands are the main enemy, because melted fat soaks into the flour and makes the baked crust greasy and hard. If the mixture starts to look oily, chill the bowl for 10 minutes before continuing, or use a food processor in short pulses.