Theromovalve switch (cooling fan) on L reg astra diesel

How does this valve work, There are three contacts on the sensor - I
would have expected the switch to have 2 contacts i.e normally open and
it closes when hot to make a contact for the cooling fan.

But it has 3 wires (all quite thick and heavy duty) - is one a 'power
feed' and the other two are connected to this feed at different
temperatures to make a dual speed fan or something?

The Haynes manual says connect the terminals together on the block to
test the fan - so I assumed it means join all 3 together (would have
been nice of they said!). So I did this and the fan ran with the
ignition on but burnt out the (v.thin) wire between two of the wires -
the fan still ran with out that connection.

With a multimeter I tested the connection between the 3 pins in the
harness and two connect directly (with no resistance I could pick up)
to one brown wire going to the fan.

Cheers - Ian