[Roy M]

It's a separate reservoir (located on the right of the luggage compartment looking in on RHD cars) which is filled with high strength screenwasher fluid. Button on the dash (I'm not even going to try and say where it would be on a LHD car) pumps this strong fluid through washers instead of highly diluted fluid which normally comes through them - cleans windshield better.

When you dismantle your washers you will see they are, in fact, two parallel systems, one for normal and one for intensive, with their own valves etc.