You move both arms up and down, but apparently they're really difficult to use. You wouldn't want to buy one to play music on, and yet I expect those who were looking for a theremin to make sound effects on wouldn't want to spend all that money on a vintage Maestro.
The seller really needs a vintage electronics collector.
In a shameless advert however, my theremin is faring annoyingly well at Ebay. It's already passed it's reserve, so I might as well promote it as much as possible. It's a pretty good price for people in Europe:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=7362605958
But anyway. Back to Maestro.