How does this type of drain off work?

[TeaAndBiscuits]

I've never seen one of these in the flesh. How do you open the drain off?
Is that a plastic cap that you pop off to reveal an allen hex inside or something?

 

[terryplumb]

Key into the bottom of the vertical bit( to the left of the threaded tail)
Square shaped ,like a radiator bleed valve .

 

[TeaAndBiscuits]

OK so presumably you have to start it draining before you can attach a hose to it then?

 

[terryplumb]

Turn both rad valves off. Loosen ,or remove the drain valve plug, open the air bleed valve ( at the top of rad ) and water then flows out the drain.
It's for draining a radiator primarily .

 

[sbgasman]

Small flat screwdriver and flip the white plastic cover off the top and there is a valve underneath which you undo with a rad bleed key

 

[terryplumb]

Apologies ,just had a closer look at the pic ,after reading sbgasmans post ,he is correct . I didn't notice the little white cap ,thought you were referring to the white cap over the lockshield.