It's on a ratchet of some sort, you bring it down, then up again into the position you want and it 'locks'.

 

Hmm, not the one I saw today. I'm speccing up a new Rapid and I'm tempted to add one, but it would be useless if like the one today.

You have to lower the handbrake to the off position and take the armrest all the way down and back up again; caught me out, as I don't remember the early versions needing this procedure but I could be wrong.

 

 

TP