How does this type of drain off work?

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?

 

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

 

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

 

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 .

 

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

 

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.

 

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