Until 1 January 2001, the use of siphon-type cisterns was mandatory in the UK but after that date the regulations additionally allowed pressure flushing cisterns and pressure flushing valves (though the latter remained forbidden in houses). These valves can sometimes be more difficult to operate than a "flapper"-based flush valve because the lever requires more torque than a flapper-flush-valve system. This additional torque is required at the tank lever because a certain amount of water must be moved up into the siphon passageway in order to initiate the siphon action in the tank. Splitting or jamming of the flexible flap covering the perforated disc can cause the cistern to go out of order.