Jan Larsen wrote:

This may seem like rather daft question but how on earth does one indeed
operate this device?

Similar in appearance to http://www.plumbworld.co.uk/1847-134

I tried to turn the square bit, nothing, then turned the collar thing
underneath the square bit, got a bit of a dribble but not much, after a
few more turns I noticed water emerging from underneath the collar and
in fear of water emerging at considerable speed in directions other than
my original intention and the swift administration of the mother of all
bollockings by the wife I decided it best to turn the collar the other
way and ask first.

I can only assume something needs to be attached to it.

A sponge or rag held up to it will protect the wall and carpet if you
are just bleeding a radiator vent. Leave the spanner on the screw so
you can close it without removing the rag, when water starts to come
through.

IIRC you need to turn the pump off to do this. Or was it have the pump
running. You get two choices.

If you are draining a radiator or something you obviously need to get a
vessel under it. You might get a length of piping from a plumbers
supplier or even a caravan shop or some such.