Shortbread-type biscuits rely on fat (eg butter) to give them their crumbly texture. The fat coats the flour particles, preventing them from absorbing moisture. This reduces gluten development in the flour, which would cause the dough to become elastic, ie stretchy. When the dough is mixed, it is soft and crumbly and has to be pressed into moulds. And once baked it has a crumbly texture and buttery flavour.
To make Rich Tea type biscuits, the formation of gluten is allowed to develop by reducing fat and adding more water - resulting in an elastic, soft dough. The dough cannot be pressed into moulds as it is too elastic, so is flattened and cut using a metal cutter. These biscuits are far less crumbly and have a crunchy texture. Roy Ballam, British Nutrition FoundationReuse content