A roller furler lets you set and reef the headsail from the cockpit. It has a drum at deck level, an aluminium foil over the forestay with a groove the sail feeds into, and a swivel at the top that the halyard pulls on. Pull the furling line and the whole foil turns, rolling the sail round the stay.
Because it's built round the forestay, a furler fault can become a rig fault.
What a surveyor checks
Furl and unfurl the sail and watch the foil. Riggers interviewed by Practical Sailor said most problems start at the top swivel, which is out of reach and gets less attention. A seized swivel twists the foil into a barber pole. Check the halyard angle at the top: Practical Sailor warns that a halyard close to parallel with the stay can wrap round it and lead to forestay failure. At deck level, check the drum turns freely and the foil joints are closed.
In the report
"Headsail furler, foil twisted in the upper third when furled under load, top swivel stiff when turned by hand from the deck. Halyard leads close to parallel with the forestay. Service or replace the top swivel and fit a halyard restrainer." See roller furler faults.