Think of a relay as a powered switch.

there are 2 sides to the relay 1 being low current (swiching circuit) and 1the other being high current (load bearing power supply)

A relay has 2 power inputs. (1 low current and 1 high current).

it has 2 outputs 1 earth and 1 high current power.
(I cant remember the numbered poles but they are 35, 37, 85 87??)

when you operate a relay, what happens is this.

your low current circuit when it is powered up and earthed moves a switch which causes the high current output to power up.

your ON/OFF switch can be on either the positive or earth side of this circuit. this is on t the low current side.

now your "high current" side gets wired to a source of power and the output side gets wired to your device to be powered (ie fan).

so as wardy said earlier, in your case you are just moving your source of power from one place to another.

Hope this makes sense... Im not a sparky I've just had a bit of experience re-wiring my projects over the years.

Cheers

Ed