The Medway Handyman explained :

When supplied by the mains it can't use more water - it can only use up to
the maximum flow of the mains supply - typically HPC's use 80% less water per
task.

Oh yes it can.... The mains water pressure is 'pushing' against the
atmosphere reducing it's flow rate. Remove the atmospheric pressure
(suck) and much more water will flow. That is how a fire engine gets
much more water out of a hydrant, than you would expect.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk