What is a flow control device for espresso?

A flow control device (FCD) replaces or supplements the standard group head paddle to allow adjustment of water flow rate during extraction — independent of pressure. Lower flow rates at the start of extraction gently pre-infuse and then ramp up. Popular on machines like the Lelit Bianca and as an aftermarket add-on. Flow control is an advanced tool for enthusiasts who want to profile extraction precisely; most home baristas don't need it until they've mastered the basics.